Washington (CNN) -- A top U.S. general said Thursday he doubts that Libyan rebels could push into the nation's capital and topple Moammar Gadhafi, even with the NATO air cover.

"I would assess that as a low likelihood," said Gen. Carter Ham, the head of U.S. Africa Command. He answered questions about the opposition forces in testimony before the Senate Armed Services Committee.

Ham also said that there would have to be "a better understanding of exactly who the opposition force is" before arms could be supplied to the rebels.

Assurances are needed "that those U.S.-provided weapons would not fall into the hands of extremist organizations," he said.

"My recommendation would be that we should know more about who they are before we make any determinations to arm them," Ham said.

He said he agrees that the Gadhafi regime has weakened after at one point closing in on victory.

"The important part is that, I think at this point, the regime has a significantly degraded ability to continue to attack civilians -- but if I may, with the notable exception of Misrata," a city in western Libya.

"The opposition forces have held an area in the northeastern portion of the city, in the port. And frankly, the port has been operating to get some relief," he said. "But the regime forces are and remain active in the city against civilians."