WASHINGTON (REUTERS) - The United States (US) government formally acknowledged on Wednesday for the first time that it had killed four Americans in counterterrorism operations in Yemen and Pakistan, where the United States has carried out regular drone strikes.

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

The letter defends the 2011 killing of militant cleric Anwar al-Awlaki, who it concedes was an intended target of a drone strike.

The three others killed were Awlaki's teenage son Abdulrahman, Samir Khan, an American of Pakistan origin, and Jude Kenan Mohammed, who had been indicted on US terrorism charges in 2009 and died in Pakistan.

Mr Holder did not specify in the letter how the Americans were killed but US officials have previously acknowledged that three of them were killed in drone strikes.