Eric Holder
© APAttorney General Eric Holder gestures while testifying on Capitol Hill in Washington. Four American citizens have been killed in drone strikes since 2009
A letter from US attorney-general confirms Barack Obama administration deliberately killed US nationals abroad.

The US has killed four of its own citizens in drone strikes in Yemen and Pakistan, the Barack Obama administration has formally acknowledged.

Eric Holder, the US attorney-general, said in a letter addressed to congressional leaders on Wednesday that three of those killed were not targets of the strikes involving drones in Yemen and elsewhere.

Holder named the four dead US citizens in a letter to members of Congress one day before President Obama is scheduled to deliver an address on the use of drones.

The letter defends the killing of Anwar al-Awlaki, the one dead American who was an intended target of a drone strike in September 2011, the letter said.

"The decision to target Anwar al-Awlaki was lawful, it was considered, and it was just," he wrote.

"The [Obama] administration is determined to continue these extensive outreach efforts to communicate with the American people," the attorney-general wrote.

The letter appears to be an effort to respond to criticisms against the Obama administration over a perceived lack of transparency, particularly concerning its "counterterrorism" abroad.