Gen. David Petraeus will be nominated to take over as CIA director, and Lt. Gen. John Allen will succeed Petraeus as head of the US-led international military force in Afghanistan, a senior administration official told reporters. In addition, longtime diplomat Ryan Crocker will be nominated as the next US ambassador to Afghanistan, the official said.

President Barack Obama will formally announce the changes on April 28, according to the senior administration official. It is the biggest reshaping of Obama's national security team since he took office in January 2009 and reflects a longstanding plan by Gates to step aside this year.

More changes also are coming. Both Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, and FBI Director Robert Mueller are not eligible for renomination for terms that expire this year.