U.S. Defense Secretary Robert Gates on Thursday told the House Armed Services Committee that no one will be able to predict how long the military mission in Libya will take.

Libyan Rebels fire rocket in the front line on the road between Ajdabiyah and Brega as forces loyal to Muammar Gaddafi threatened to push them further away from strategic oil refineries in eastern Libya March 31, 2011. Rebels massed outside Brega on Thursday said their forces were still fighting Muammar Gaddafi's troops for control of the east Libya oil town. [asser Nouri/Xinhua]

Gates said that as NATO takes control of the military mission, the United States will significantly "ramp down" its role in the operation. Exceptions would include providing unique capabilities that others cannot match in both kind and scale, such as electronic warfare, aerial refueling, and intelligence.

Gates also told lawmakers that other nations rather than the United States should provide training or assistance to Libyan rebels, as "that is not a unique capability for the U.S. and as far as I'm concerned somebody else should do that."

The defense secretary's comments came as Libyan leader Moammar Gaddafi's forces struck forcefully back at the rebels this week, recapturing lost ground and triggering pleas for help from the battered and failing opposition forces.

The defense secretary and Mike Mullen, chairman of the Joint Chiefs of Staff, were appearing Thursday before the House and Senate Armed Services Committee in the wake of new revelations that small teams of CIA operatives are working in Libya.

Gates declined to comment on the CIA report, and told lawmakers President Barack Obama has no additional military moves in his mind beyond the current one.

Gates said the cost of the military mission until Monday was 550 million dollars, and the cost for the administration after it takes on a more supporting role will be approximately 40 million.

Mullen, for his part, said NATO-led strikes have knocked out about 20 percent of Libyan government forces, but from a military point of view, the Libyan army is not "about to break."

The White House is taking heat from Congress on its ambiguity regarding its involvement in the Libyan mission. Lawmakers are concerned about the possibility that the United States would be embroiled in another country's civil war, and they are unhappy about the mission's cost and the fact that they were not consulted before the operation, as Congress wields the exclusive right to declare wars.