Drone strikes to remain linchpin of US anti-terror efforts: Obama

President Barack Obama on Thursday for the first time outlined a change in perspective on the US war on terror, shifting the focus from large international terrorist groups or nations harbouring them to a narrower threat from smaller networks and home-grown extremists.

In a lengthy address at the National Defense University, Obama nonetheless defended his controversial drone strikes programme and said it would continue to be a linchpin of the US response to the evolving dangers.

He made his most vigorous public defence yet of drone strikes as legal, effective and necessary to counter terror threats progress. He pledged to allow greater oversight of the drone programme, but said he plans to keep the most lethal efforts with the unmanned aircraft under the control of the Central Investigation Agency (CIA).

Obama implored Congress to close the much-maligned Guantanamo Bay detention centre in Cuba.

He also argued that changing threats require changes to the nation's counterterrorism policies.

"Neither I, nor any president, can promise the total defeat of terror," Obama told his audience of students, national security and human rights experts and counterterror officials. "What we can do - what we must do  - is dismantle networks that pose a direct danger, and make it less likely for new groups to gain a foothold, all while maintaining the freedoms and ideals that we defend."

Obama's address came amid increased pressure from Congress on both the drone programme and the status of the Guantanamo prison. A rare coalition of bipartisan lawmakers has pressed for more openness and more oversight of the highly secretive targeted strikes, while liberal lawmakers have pointed to a hunger strike at Guantanamo in pressing Obama to renew his stalled efforts to close the detention centre.

The president cast the drone programme as crucial in a counterterror effort that will rely less on the widespread deployment of US troops as the war in Afghanistan winds down. But he acknowledged the targeted strikes are no "cure-all" and said he is deeply troubled by the civilians unintentionally killed.

"For me, and those in my chain of command, these deaths will haunt us as long as we live," he said. Before any strike, he said, "there must be near-certainty that no civilians will be killed or injured - the highest standard we can set."

In Pakistan alone, up to 3,336 people have been killed by the unmanned aircraft since 2003, according to the New America Foundation which maintains a database of the strikes. However, the secrecy surrounding the drone programme makes it impossible for the public to know for sure how many people have been killed in in strikes, and of those, how many were intended targets.

In an attempt to lift the veil somewhat, the Justice Department revealed for the first time Wednesday that four Americans had been killed in US drone strikes abroad. Just one was an intended target - Anwar al-Awlaki - who officials say had ties to at least three attacks planned or carried out on US soil.

The other three Americans, including al-Awlaki's 16-year-old son, were unintended victims.

Drones aside, some Republicans criticized Obama as underestimating the strength of al-Qaida in his speech and for proposing to repeal the president's broad authorization to use military force against the nation's enemies - powers granted to George W Bush after the 11 September 2001 attacks.

Obama announced new "presidential policy guidelines" on the standards his administration uses when deciding to launch drone strikes.

According to an unclassified summary of the guidelines, the US will not strike if a target can be captured, either by the US or a foreign government; a strike can be launched only against a target posing an "imminent" threat, and the US has a preference for military control of the drone program.

However, the CIA will continue to work with the military on the program in Yemen, and control it in Pakistan, given the concern that al-Qaida may return in greater numbers as US troops leave Afghanistan.

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.

Obama's advisers said the new guidelines would effectively limit the number of drone strikes in terror zones and pointed to a future decline of attacks against extremists in Afghanistan as the war ebbs. But strikes elsewhere will continue. The guidelines will apply to strikes against both foreigners and US citizens abroad.