US General David Petraeus, who is in charge of US forces in Iraq and Afghanistan, has briefly collapsed at a Capitol Hill hearing.

It is thought the episode, from which he shortly regained his composure, was caused by dehydration.

Petraeus, a fitness fanatic who was treated for prostate cancer last year, said he became "light headed" during the Senate Armed Services Committee hearing and was a little dehydrated.

"It wasn't Senator McCain's questions," he joked later, referring Arizona Senator John McCain who was questioning him at the time over the pace of military action in Afghanistan.

Petraeus, 57, appeared to slump over his seat before being helped by aides and walking out of the room.

Afterwards he told reporters that he felt better after a couple of bananas.

The hearing was adjourned and will resume on Wednesday.

"He's eating. He probably didn't have enough water to drink coming in here this morning. But he appears to be doing very much better," Senator Carl Levin, chairman of the Armed Services Committee, told the hearing.