US President Barack Obama has signed a new policy directive putting into law guidelines that only allow drone attacks to be used to prevent imminent attacks, and when a suspect cannot otherwise be captured.

There also must be a "near certainty" that no civilians would be killed.

American deaths

On Wednesday it was revealed that the US has killed four Americans in unmanned drone attacks since 2009 in Pakistan and Yemen, but officials said only one of those deaths - that of radical cleric Anwar al Awlaki - was intentional.

The President said he declassified this information in the interest of transparency and to dismiss "outlandish" theories about policies on targeting US citizens.

In his address on Thursday he said Americans cannot use their citizenship as "a shield" to wage attacks on other Americans. However, he added that no terror target, US citizen or otherwise, would ever be targeted - "with a drone or with a shotgun" - without due process.

Guantanamo Bay

Speaking from National Defense University at Fort McNair in Washington DC, the president also called on Congress to change its policies on prisoners held at Guantanamo Bay, Cuba.

He said he will lift a 2009 ban on transferring detainees from the US base at Guantanamo Bay, Cuba, to Yemen. Roughly 90 of the 166 detainees there are Yemeni.

At the same time, terror suspects will be brought to justice in US courts and the military justice system where appropriate.

In his lengthy address Mr Obama indicated he regrets uses of torture and unlawful detention to gather terror intelligence since the 9/11 attacks, and said he is "haunted" by unintended civilian casualties in unmanned drone strikes.

But the president also said the US is safer because of his administration's targeted and continued work to combat terrorism, and insisted US actions have been legal.