President-elect Obama has hired former Senate Armed Services Committee chairman Sam Nunn to help shepherd his Pentagon transition, a spokeswoman said Tuesday.

Nunn, a former Georgia senator and veteran Democratic defense adviser, was once rumoured as a potential running mate for Obama. Transition spokeswoman Stephanie Cutter said Nunn will perform "an informal senior adviser role throughout the defense transition process."

Nunn's role has been described by others, speaking anonymously because the transition teams have not been announced, as the leader of Obama's defense transition. Similarly, a senior administration official said former Secretary of State Warren Christopher would advise Obama on his State Department transition.

Obama's national security transition offices will work with the outgoing Bush administration to hand off management of two wars and complex diplomacy with North Korea, Russia and other nations. Obama has not announced his choices to be defense secretary and secretary of state.

Nunn and Christopher would be part of a national security brain trust for Obama that is heavy on former Clinton administration officials, including possible national security adviser James Steinberg, a former State Department official.

At the Pentagon, the large transition office would likely include former Clinton Navy Secretary Richard Danzig and former Clinton-era Pentagon comptroller Bill Lynn, officials said. The announcements are expected later this week.

"There's a lot of disinformation out there," Cutter said. "We're working hard to put the agenda review teams together and expect they'll be announced and inside the agencies by the end of the week."

Clinton Pentagon officials turned think tankers Michele Flournoy and Kurt Campbell are also mentioned as part of the Obama transition office at the Pentagon. Clinton-era State Department officials Wendy Sherman and Tom Donilon are mentioned in similar roles at State.