As technical failures bedevil the rollout of President Obama’s health care law, evidence is emerging that one of the program’s loftiest goals — to encourage competition among insurers in an effort to keep costs low — is falling short for many rural Americans.

While competition is intense in many populous regions, rural areas and small towns have far fewer carriers offering plans in the law’s online exchanges. Those places, many of them poor, are being asked to choose from some of the highest-priced plans in the 34 states where the federal government is running the health insurance marketplaces, a review by The New York Times has found.

Of the roughly 2,500 counties served by the federal exchanges, more than half, or 58 percent, have plans offered by just one or two insurance carriers, according to an analysis by The Times of county-level data provided by the Department of Health and Human Services. In about 530 counties, only a single insurer is participating.

The analysis suggests that the ambitions of the Affordable Care Act to increase competition have unfolded unevenly, at least in the early going, and have not addressed many of the factors that contribute to high prices. Insurance companies are reluctant to enter challenging new markets, experts say, because medical costs are high, dominant insurers are difficult to unseat, and powerful hospital systems resist efforts to lower rates.

“There’s nothing in the structure of the Affordable Care Act which really deals with that problem,” said John Holahan, a fellow at the Urban Institute, who noted that many factors determine costs in a given market. “I think that all else being equal, premiums will clearly be higher when there’s not that competition.”

The Obama administration has said 95 percent of Americans live in areas where there are at least two insurers in the exchanges. But many experts say two might not be enough to create competition that would help lower prices.

For example, in Wyoming, two insurers are offering plans at prices that are higher than in neighboring Montana, where a third carrier is seen as a factor in keeping prices lower.

It is unclear how the online marketplaces might evolve over time. Many large insurers are closely watching what happens in the first year to decide whether to more aggressively pursue new markets. In the meantime, problems with the healthcare.gov Web site are making it harder for them to know whether the exchanges’ slow start is the result of technical difficulties or more serious underlying problems, such as a lack of consumer demand, that would discourage them from entering.

In some cases, competition varies markedly across county lines. In Monroe County, Fla., which includes the Florida Keys, two insurers, Cigna and Florida Blue, offer plans on the federal exchanges. In neighboring Miami-Dade County, there are seven companies, including Aetna and Humana, two of the nation’s largest players.

In rural Baker County, Ga., where there is only one insurer, a 50-year-old shopping for a silver plan would pay at least $644.05 before federal subsidies. (Plans range in price and levels of coverage from bronze to platinum, with silver a middle option.) A 50-year-old in Atlanta, where there are four carriers, could pay $320.06 for a comparable plan. Federal subsidies could significantly reduce monthly premiums for people with low incomes.

Counties with one carrier are mostly concentrated in the South. Nearly all of the counties in Mississippi and Alabama, for example, are served by just one insurer, according to The Times’s analysis. Other states with scarce competition include Maine, West Virginia, North Carolina and Alaska.

“The consumer wants some level of choice,” said Alexander K. Feldvebel, the deputy insurance commissioner for New Hampshire, where one carrier, Anthem Blue Cross, owned by WellPoint, now offers plans. “You don’t have that when you have a single carrier offering all the products.”

The Times examined carriers and prices on the federal exchanges for the second-cheapest silver plan, the level on which subsidies are based, available to a 50-year-old. Comparable data for state-run plans was unavailable.

The Obama administration, while not disputing the findings, responded to the analysis in a statement that the marketplaces “allow insurers to compete for customers based on price and quality.” It added that the tax-credit subsidies that will lower monthly payments for many consumers had also “brought more companies to the market, resulting in increased options for consumers and lower-than-expected premiums.”

Insurance executives say they set their rates without knowing what other insurers were doing.

“No one knew who was going to file,” said Barbara Morales Burke, an executive with BlueCross BlueShield of North Carolina, the only insurer offering coverage in 61 of the state’s 100 counties. “We developed the rates we always do based on actuarial information and reasonable estimates.”

Market Concentration

The Affordable Care Act, which was passed in 2010, was designed to make health insurance available to people who had not been able to afford it or had been denied coverage because of pre-existing conditions. It has transformed the market for individual insurance by creating marketplaces aimed at making it easier for consumers to compare their options. The law also sought to level the playing field for new insurers.

Before its passage, the existing insurance marketplace was often dominated by a single insurer.

“The picture that comes away even before the A.C.A. went into effect was that insurance markets are highly concentrated in many states,” said Larry Levitt, a policy expert at the Kaiser Family Foundation.

One of the main ways of fostering competition was through the creation of consumer-operated plans, called co-ops, to compete with existing insurers. They received some $2 billion in federal loans and are operating on 22 exchanges. At least 18 others were proposed when the program was discontinued as part of last year’s negotiations over the fiscal cliff.

Concerns have risen recently about the co-ops’ financial viability because of heavy regulation and a lack of visibility so far among consumers, although it is too early to know whether or not they will succeed.

“If co-ops are the game-changing, paradigm-changing force that we hope and expect them to be, they will permanently drive down rates,” said John Morrison, the president of the board of the National Alliance of State Health CO-OPs, which recently released a study concluding that premiums were lower in states with co-ops.

Some say the arrival of a co-op changed the landscape in Montana, where the insurers Blue Cross and PacificSource were joined by Montana Health CO-OP.

In neighboring Wyoming, two insurers are offering plans under the exchange: Blue Cross and WINHealth, a small health maintenance organization, or H.M.O. The cheapest silver plan available to a 50-year-old in Wyoming cost nearly as much as the most expensive Montana plan.

“Adding that third competitor really changes the landscape vastly,” said Jerry Dworak, chief executive of the Montana co-op. He said the other insurers had predicted that their rates would be 25 percent higher in the marketplace, but those increases did not materialize. “It was amazing how close the rates were,” he said.

The story is the same in other states, like South Carolina, where a new co-op competes in many rural areas.

“If the co-op didn’t exist, we would look like North Carolina,” said Jerry Burgess, the chief executive of Consumers’ Choice Health Plan.

Some insurers, especially those that specialize in serving Medicaid populations, have seen opportunity in the millions of new customers expected to enroll in the marketplaces. Some hospital systems have also created their own plans. About a quarter of the insurers are new to the individual market.

Another effort to increase competition has been less successful. The law created what are called multistate plans, in which a private carrier offers insurance in the marketplaces of multiple states under contract with the federal government. But federal officials selected Blue Cross to offer those plans, which is already the dominant insurer in many states.

“If you’ve got Blue Cross competing with Blue Cross, it doesn’t give you much competition,” said Timothy S. Jost, a law professor at Washington and Lee University.

In Orange County, Ind., the silver plan offered through Anthem Blue Cross and Blue Shield’s multistate plan is the same price — $487.11 for a 50-year-old — as another Anthem silver plan offered in the marketplace.

The Rural Problem

In rural regions, several factors combine to create a landscape that is inhospitable to newcomers. Developing relationships with doctors and hospitals can be costly where cities and towns are widely scattered and the pool of potential customers is small.

“I think the problem was that the Affordable Care Act was designed for where the majority of the people live, in the big cities where there’s a lot of competition among health care providers,” said Tom Hirsig, Wyoming’s insurance commissioner.

He said insurers simply did not find his state, with its population of fewer than 600,000, attractive.

“You’ve got to have some bargaining chips and we don’t have that much,” he said.

Often a single hospital dominates an area, giving insurers little leverage when negotiating reimbursement rates. Only one Wyoming county is served by more than one hospital, said Stephen K. Goldstone, the chief executive of WINHealth.

“What it costs to be treated here is more expensive than other places because there’s no competition among providers,” Mr. Goldstone said.

In southwest Georgia, another rural region, Blue Cross and Blue Shield of Georgia is the dominant carrier, and it is the only insurer operating in 54 of the state’s 159 counties.

“This has been what Georgia’s issues have been, that rural areas don’t have the best access to care,” said Amanda Ptashkin of Georgians for a Healthy Future, a consumer advocacy group.

Bert Kelly, a spokesman for Blue Cross and Blue Shield of Georgia, which is owned by WellPoint, said the higher premiums reflected the area’s higher medical costs and not a lack of competition.

In some areas, having one or two major carriers may be an advantage in being able to negotiate with powerful hospital systems.

Mr. Feldvebel, the New Hampshire regulator, said, “The bigger your carrier is, the bigger the discount the carrier can deliver because they have more lives to bargain with.”

It is also difficult to attract new insurers to areas where the population has health problems. Only one carrier, Highmark Blue Cross, is offering coverage in West Virginia, which has high rates of obesity and chronic diseases like diabetes.

Spreading Blame

A lack of competition does not always translate to higher premiums. In Tennessee, much of the state is served by just one or two carriers, but premiums are lower there than in neighboring states, even though Tennessee also struggles with high rates of obesity and chronic diseases.

“We smoke and we eat and we use prescription drugs far above what national averages are,” said Brian Haile, who was in charge of planning Tennessee’s state-run insurance marketplace before the governor decided to switch to federal oversight late last year.

Mr. Haile said he found the lack of competition in the online marketplace in his state “shameful” and blamed the federal government. Still, he said he believed his earlier efforts to encourage carriers to reduce rates worked.

Some regulators blame state lawmakers for not taking a more active role. In North Carolina, lawmakers decided not to expand Medicaid eligibility and not to run their own marketplace. Insurers “are accustomed to working with state insurance regulators,” rather than federal officials, said Wayne Goodwin, the state’s insurance commissioner.

“Had North Carolina maintained a state-based exchange and if it had expanded Medicaid, we would have had more health insurance carriers offering choices for consumers,” said Mr. Goodwin, an elected Democrat.

Observers cautioned against drawing too many conclusions from the current landscape, noting that several major insurers were waiting to see what happens next.

One such company is Centene, a national insurer that has focused on plans for Medicaid recipients and low-income consumers.

K. Rone Baldwin, a Centene executive, said the company had offered plans under the brand name Ambetter Health in nine states, but it views this year as merely a start.

“We don’t view 2014 as the make-or-break year,” he said.