Libyan rebels have begun an assault on an oil refinery in a city just west of Tripoli, in a bid to drive out forces loyal to Moammar Gadhafi and continue their advance toward the capital.

The refinery at Zawiya, 50 kilometers from Tripoli, is one of the few sources of fuel for Mr. Gadhafi's troops and people in the capital. A rebel commander says the pipeline linking the refinery to Tripoli has been shut off, but that could not be confirmed.

Reporters in Zawiya say the rebels control a good part of the city. Pro-Gadhafi forces shelled it again Wednesday as snipers at the refinery fired at rebel fighters.

On Tuesday, the rebels dismissed reports that they held talks with aides to the Libyan leader.

The head of the rebel Transitional National Council, Mustafa Abdel Jalil, said there have been no direct or indirect talks with Mr. Gadhafi's representatives, and that such talks are “unthinkable” unless the Libyan leader steps down.

Western news reports had said Mr. Gadhafi's aides met this week with Libyan rebels and U.N. envoy Abdul Ilah al-Khatib at a hotel on the Tunisian island of Djerba.

In Washington Tuesday, U.S. officials said Mr. Gadhafi's days are “numbered,” as rebels reported capturing towns to the south and west of Tripoli.