Eric Holder acknowledges previously classified details of drone program and says US deliberately targeted Anwar al-Awlaki, who died in Yemen in 2011

The White House has launched a new effort to draw a line under its controversial drone strike policy by admitting for the first time that four American citizens were among those killed by its covert attacks in Yemen and Pakistan since 2009.

In a letter to congressional leaders sent on Wednesday, attorney general Eric Holder acknowledged previously classified details of the drone attacks and promised to brief them on a new US doctrine for sanctioning such targeted killings in future.

Holder claimed one of the US citizens killed, Anwar al-Awlaki, was chief of external operations for al-Qaida in the Arabian Peninsula (Aqap) and had been involved in plots to blow up airplanes over US soil. However, Holder said three others killed by drones – Samir Khan, Abdul Rahman Anwar al-Awlaki and Jude Kenan – were not "specifically targeted". The second of these victims, Anwar al-Awlaki's son, is said by campaigners to have been 16 when he died in Yemen in 2011.

The Bureau of Investigative Journalism estimates that between 240 and 347 people have been killed in total by confirmed US drone strikes in Yemen since 2002, with a further 2,541 to 3,533 killed by CIA drones in Pakistan.

Amid mounting concern that the policy has harmed US interests overseas, President Obama is expected to give a major speech on his counter-terrorism strategy at the National Defense University in Washington on Thursday, marking the start of a concerted effort to better justify and explain the killings.

"The president will soon be speaking publicly in greater detail about our counterterrorism operations and the legal and policy framework," Holder told 22 senior members of Congress in Wednesday's letter.

"This week the president approved a document that institutionalises the administration's exacting standards and processes for reviewing and approving operations to capture or use lethal force against terrorist targets outside the United States and areas of active hostilities."

The attorney general said this document would remain classified, but relevant congressional committees would be briefed on its contents. No further details were given of other killings in the five-page letter.

Earlier, White House spokesman Jay Carney said Obama would also outline his renewed attempt to shut the Guantánamo Bay detention centre in the speech and seek to explain why previous efforts had failed.

After a week in which Obama has been accused of failing to deal openly with crises such as the the targeting of Tea Party activists by the Internal Revenue Service, the White House hope it can defuse concern over drones and Guantánamo by being more transparent about its objectives.

"These are matters that … he believes are subject to legitimate questions, and that these are issue areas he believes we need to be as transparent as possible about." said Carney. "And I think you'll see that reflected in his remarks tomorrow."

The White House says Thursday's speech will cover "broad counter-terrorism policy, including military, diplomatic, intelligence, and legal efforts".

"[Obama] will review the state of the threats that we face, particularly as the al-Qaida core has weakened but new dangers have emerged," it added. "He will discuss the policy and legal framework under which we take action against terrorist threats, including the use of drones.

"He will review our detention policy and efforts to close the detention facility at Guantánamo Bay. And he will frame the future of our efforts against al Qaeda, its affiliates and its adherents."

Greater transparency is unlikely to satisfy critics of the drone strikes alone, but the White House has also been keen to stress in recent days that the number of attacks has fallen significantly since Obama's first term and Thursday's speech may mark a turning point in the use of such tactics by the US.