United States President Barack Obama says he has drawn up new guidelines for the use of unmanned aircraft to hit militant targets abroad.
In a major speech on counter-terrorism at the National Defense University in Washington, he stressed they should be used only when faced with an imminent threat and when there was near-certainty that no civilians would be affected.
With greater frankness than ever before, the president on Thursday disussed his administration's use of unmanned drone strikes to kill members of al Qaeda and its affiliates, the BBC reports.
"This new technology raises profound questions about who is targeted and why, about civilian casualties and the risk of creating new enemies, about the legality of such strikes under US and international law."
Mr Obama has drawn up a new set of guidelines for drones. He didn't give many details, but said they could only be used in the context of what he called continuing imminent threats to Americans and where there was near certainty that no innocent civilians would be affected.
He also defended the use of drones to kill four US citizens.
"We are at war with an organisation that right now would kill as many Americans as they could if we did not stop them first. So this is a just war - a war waged proportionally, in last resort, and in self-defence."
Human rights groups have long condemned the use of unmanned drones to carry out killings.
The president also outlined measures aimed at closing the US detention centre in Guantanamo Bay in Cuba.
Mr Obama said he hopes more Yemeni detainees will be transferred to their home country, while efforts would be made to put on trial and imprison others in the US.
The Democratic president made shutting the prison a top priority at the beginning of his first term, but his effort foundered amid strong bipartisan opposition in Congress.
Mr Obama told his audience: "Guantanamo has become a symbol around the world for an America that flouts the rule of law."
The president said he was appointing envoys from the defence and state departments to negotiate transfers of detainees to other countries, and said he would lift a moratorium on transfers to Yemen.
After the speech, Republican Senator Saxby Chambliss told reporters that Mr Obama was wrong to lift the Yemen moratorium because Yemeni authorities could not be trusted to "handle them".