United States defence secretary Robert Gates has made a passionate case for intervention in Libya despite admitting there is little understanding of who the rebels are.

Mr Gates would not address reports that CIA operatives are already inside Libya, but the claim has revived debate about the promise not to put American boots on the ground in Libya.

Mr Gates and top uniformed military officer Admiral Mike Mullen spent a day answering questions on Capitol Hill and being pilloried for taking America into a conflict without getting approval from Congress.

Giving evidence before a congressional committee, Mr Gates stuck to the administration's line that there would be no American boots on the ground.

"We're saying we're not going to put any boots on the ground ... that is my understanding," he said.

But he then clarified his position.

"I'm pretty confident that NATO as an organisation would not authorise boots on the ground as part of this operation," he said.

The confusion prompted one US congressman to dub Libya probably the "most muddled definition of a military operation in US history".

News of a secret order signed by the US president authorising covert American support for the rebels has been received as paving the way for a possible arming of the opposition.

But the defence secretary did not want to discuss that.

"Frankly there are many countries that can do that. That is not a unique capability for the United States and as far as I am concerned, somebody else should do that," Mr Gates said.

The defence secretary conceded very little was known about the opposition, which he described as almost a misnomer because the rebels were such a disparate, scattered group.

"To be honest, other than a relative handful of leaders, we don't have much visibility into those that have risen against Gaddafi," he said.

The hearing was told the opposition had only about 1,000 fighters with military training, but the US had no contingency plan to avoid a stalemate other than keeping the pressure on the Gaddafi regime.

"We may not know much about the opposition or the rebels, but we know a great deal about Gaddafi. This guy has been a huge problem for the United States for a long time," he said.

He suggested one of Mr Gaddafi's own military could "take him out" or Libya's tribes could abandon him.

White House spokesman Jay Carney indicated the defence secretary's comments were no guarantee the US would not be arming the rebels.