The lawyer leading a UN drone inquiry has praised a speech by US President Barack Obama as a "significant step towards increased transparency".
Ben Emmerson said Mr Obama had set out more clearly than ever before the legal justifications for targeted killing.
Pakistan, the main focus of the strikes, has reiterated its view that drones are "counter-productive".
Mr Obama pledged to continue strikes, but with tighter oversight of the programme and stricter targeting rules.
Mr Emmerson, a United Nations human rights special rapporteur, launched an inquiry into drones in January, saying their use "represents a real challenge to the framework of international law".
The inquiry is examining 25 attacks, in Pakistan, Afghanistan, Yemen, the Palestinian territories and Somalia.
He said in a statement that Mr Obama's speech had broken new ground on a number of issues.
"It sets out more clearly and more authoritatively than ever before the administration's legal justifications for targeted killing, and the constraints that it operates under," he said.
"The publication of the procedural guidelines for the use of force in counter-terrorism operations is a significant step towards increased transparency and accountability."
The Pakistani foreign ministry said it appreciated that Mr Obama had acknowledged "force alone cannot make us safe" and welcomed his resolve to rebuild ties between the nations.
But the ministry added: "The government of Pakistan has consistently maintained that the drone strikes are counter-productive, entail loss of innocent civilian lives, have human rights and humanitarian implications and violate the principles of national sovereignty."
A senior official from Nawaz Sharif's Pakistan Muslim League has told the BBC the party is disappointed that President Obama gave no indication he would consult the Pakistan government about the continued use of drone attacks.
He said the question of the Americans bombing Pakistani territory without permission is the biggest foreign policy issue facing the new administration, which is preparing to take power after its recent election win.
The issue is hugely controversial in Pakistan, where parts of the government and military are often accused of ignoring or even condoning some of the drone strikes.
According to several estimates, US strikes in Pakistan hit a peak in 2010 when more than 100 drone attacks were reported.
Last year, the number was thought to be fewer than 50.
Mr Emmerson said after a trip to Pakistan in March that Pakistan "does not consent to the use of drones by the United States on its territory and it considers this to be a violation of Pakistan's sovereignty and territorial integrity".
However, correspondents say the US could not launch drone strikes without tacit support from Pakistan.
Mr Obama spelled out his new policy on drones as part of a wider speech on counter-terrorism.
"America does not take strikes to punish individuals, we act against terrorists who pose a continuing and imminent threat to the American people," he said.
And he added that the strikes were permissible only "when there are no other governments capable of effectively addressing the threat" and there must be "near certainty" that no civilians would be killed.
His speech coincided with the signing of new "presidential policy guidance" on the use of drone strikes.
The policy document curtails the circumstances in which drones can be used in places that are not overt war zones, such as Pakistan, Yemen and Somalia.
The use of unmanned drones in foreign countries has been overwhelmingly backed in US opinion polls.
However, the same polls reveal that few support the use of drones on US territory.
