Libyan rebels have launched an offensive in the mountains southwest of the capital, Tripoli, in an effort to move their front lines closer to Tripoli, Libyan leader Moammar Gadhafi's stronghold.

Rebels used heavy arms Wednesday to push into the al-Qawalish area, a government-held position near Tripoli. Witnesses said they could hear NATO warplanes overhead.

The fighting comes after at least 11 Libyan rebels were killed in clashes with pro-government forces near the opposition-held city of Misrata.

Medics and rebels said the deaths occurred late Monday and Tuesday after forces loyal to Libyan leader Moammar Gadhafi shelled targets on the outskirts of Misrata, about 200 kilometers east of Tripoli. They say more than 40 rebels were wounded.

In Washington Tuesday, the U.S. Senate postponed voting on a resolution that authorizes U.S. military involvement in Libya for up to a year, but prevents the use of American ground troops in the conflict.

The sponsors of the resolution, Democrat John Kerry and Republican John McCain , say it is important not to abandon the U.S. mission, arguing that Mr. Gadhafi is close to losing his grip on power.

The Libya resolution recently failed to pass in the U.S. House of Representatives.

Some Republican and Democratic lawmakers are angry with U.S. President Barack Obama for failing to seek congressional authorization to intervene in Libya.