奥巴马为无人机袭击辩解

US President Barack Obama says the United States is at a crossroad in terms of confronting threats against its security.

"From our use of drones to detention of terror suspects, the decisions that we are making now will define the type of nation and world that we leave to our children. So America's at a crossroads. We must define the nature and scope of this struggle or else it will define us."

In a speech delivered at the National Defense University, Obama defended the controversial drone attacks as legal, effective and a necessary linchpin in an evolving US security policy.

But he acknowledged the targeted strikes were no "cure-all" and said he was haunted by the civilians unintentionally killed.

The president also announced a renewed push to close the Guantanamo Bay detention centre in Cuba, including lifting a moratorium on prisoner transfers to Yemen.

Before that, he also signed new "presidential policy guidelines" aimed at illustrating the standards the US applies before carrying out drone attacks.

However, the CIA is still expected to maintain control of the drone programme in Yemen, as well as in Pakistan's tribal areas.