President Barack Obama has defended his country's controversial drone attacks as legal, effective and a necessary tool in an evolving US counterterrorism policy.

But addressing an audience at the National Defence University on Thursday, he acknowledged that the targeted strikes were no "cure-all" and said he was haunted by the civilians unintentionally killed.

Obama framed his speech as an attempt to redefine the nature and scope of terror threats facing the US, noting the weakening of al-Qaeda and the impending end of the US war in Afghanistan.

"So America is at a crossroads. We must define the nature and the scope of the struggle, or else it will define us," said Obama, saying that threats to diplomatic facilities must be dealt with as well as "homegrown extremists".

His speech came a day after his administration revealed for the first time that a fourth American citizen had been killed in secretive drone raids abroad.

The speech also reaffirmed Obama's 2008 campaign promise to close the military prison at Guantanamo Bay, where terror suspects have been held.

Obama said the US was committed to "capturing terrorist suspects" and prosecuting them, but that "the glaring exception to this time-tested approach is the detention centre at Guantanamo Bay".

"When I ran for president the first time, John McCain supported closing Gitmo. No person has ever escaped from one of our super-max or military prisons in the United States," he said.

"Our courts have convicted hundreds of people for terrorism-related offences, including some who are more dangerous than most Gitmo detainees."

Obama highlighted the role of the US Congress in inhibiting the closure of the detention facility.

"There is no justification beyond politics for Congress to prevent us from closing a facility that should never have been opened," he said.

Obama was heckled by a person in the audience on the issue of force-feeding hunger-striking detainees in the Guantanamo Bay prison.

Obama's speech was repeatedly interrupted by an audience member who shouted "I love my country, I love the rule of law. The drones are making us less safe".

'Harsh judgment'



The White House said on Wednesday that Obama's speech coincided with the signing of new "presidential policy guidance'' on when the US could use drone strikes.

Drafts of the guidance reviewed by counterterrorism officials gave control of drone strikes outside Pakistan and Yemen to the US military, enshrining into policy what is already common practice, according to two US officials briefed on the proposed changes.

Obama also announced a moratorium on transferring prisoners from Guantanamo Bay to Yemen, saying history would "cast a harsh judgment" on the offshore detention programme and those who failed to end it.

His plan naming a new envoy to handle the transfers and ordering the Pentagon to designate a site in the US for military trials of the detainees.

Yemen welcomed Obama's announcement. "The government of Yemen welcomes President Obama's remarks and actions today. In particular, Yemen welcomes the administration's decision to lift the moratorium on detainee transfers to Yemen," Mohammed Albasha, spokesman for the Yemeni embassy in Washington, said.

Drone controversy

Obama has pledged to be more open with the public about the scope of the drone strikes.

However, a growing number of legislators in Congress are seeking to limit US authorities that support the deadly drone strikes, which have targeted a wider range of threats than initially anticipated.

"America cannot take strikes wherever we choose," said Obama, saying that such strikes "save lives".

He acknowledged civilian deaths as "a hard fact" that will "haunt us as long as we live".

The speech comes amid growing impatience in Congress with the sweeping authority it gave the president after the September 11, 2001, attacks in light of the targeting of suspected terrorists with lethal drone strikes.

Republicans and Democrats fear that they have given the president a blank check for using military force worldwide.

Shifting the responsibility of some of the drone programme from the Central Intelligence Agency to the military has given Congress greater oversight of the secretive programme and members say they want even more.

Under the draft guidance, the CIA drone programme would remain up and running, to target al-Qaeda in Pakistan's tribal areas, with US troops drawing down in Afghanistan and concern rising that al-Qaeda might return in greater numbers to the region.

The military and the CIA currently work side by side in Yemen, with the CIA flying its drones over the northern region out of a covert base in Saudi Arabia, and the military flying its unmanned aerial vehicles from Djibouti.