troops under a U.N. mandate to protect civilians. More explosions rocked Tripoli shortly before midnight Thursday (5 p.m. ET), indicating fresh airstrikes were under way in the Libyan capital.

Earlier Thursday, the rebels said their forces had captured five towns and surrounded a sixth in the plains below the Nafusa mountain range, which borders Tunisia.

Hundreds of rebels moved from their mountain positions at dawn. With heavier weapons leading the way with lighter armed fighters following, they initially encountered fierce resistance from Gadhafi's loyalists.

Col. Jumma Ibrahim, spokesman for the Military Council for the western mountain region, said several major battles had taken place before the towns were secured. He named the captured towns as al-Ruwais, Takout, al-Jawsh, Bader and al-Ghazzaya, and the surrounded town as Umm al-Far.

Ibrahim said rebels werenow negotiating with Gadhafi troops inside Umm al-Far.

He said four rebels had been killed in the fighting and at least 20 were wounded, while 20 Gadhafi fighters had been captured.

The claimed successes come after one of the biggest rebel offensives in recent weeks and, if true, puts the fighters closer to capturing a significant supply route used by Gadhafi forces.

Ibrahim said that on the eastern side of the mountains, Gadhafi forces had been shelling rebel-held positions near Beir al-Ghanam and also near Qawalish.

Rebel fighters last month suffered heavy casualties in a failed attempt to wrest control of al-Ghazzaya from forces loyal to Gadhafi.

Last week, rebel fighters manning hilltops in the western mountains overlooking al-Ghazzaya told CNN that they had been watching the government forces reinforcing their weapon stocks