WASHINGTON — As President Barack Obama prepared to deliver a major speech on national security Thursday, his administration acknowledged for the first time that it had killed four U.S. citizens — one more than previously known — in drone missile strikes in Yemen and Pakistan.

The disclosure Wednesday raised fresh questions about the secret drone campaign, a signature part of Obama's counterterrorism effort, in which several thousand suspected terrorists, militants and others have been killed. The White House has insisted the targeting is precise and causes few accidental casualties.

In a letter to congressional leaders, Attorney General Eric H. Holder Jr. said the administration had deliberately targeted Anwar al-Awlaki, a radical Muslim cleric who was born in New Mexico and killed in Yemen in September 2011, and had killed three other Americans who were not targeted.

They include Samir Khan, an al-Qaida propagandist who grew up in the New York City borough of Queens and who was killed alongside al-Awlaki, and al-Awlaki's 16-year-old son, Abdulrahman, who was born in Denver and killed in Yemen two weeks after his father. All three deaths were reported by the news media, but the administration had not previously admitted its role publicly.

The fourth American was Jude Kenan Mohammad, killed in northwestern Pakistan in November 2011. His death remained so secret that he was still listed Wednesday as wanted by the FBI. Mohammad, who grew up near Raleigh, N.C., traveled to Pakistan in 2008 "to engage in violent jihad," according to a 2009 federal indictment in North Carolina.

The development raises the stakes for a long-awaited policy speech Obama is to give Thursday at the National Defense University. He faces growing pressure to explain the rationale for secret drone strikes, especially against U.S. citizens, and to outline his strategy against a weakened al-Qaida.