TRIPOLI (Agencies) - Forces loyal to Col Gaddafi have launched further air strikes on the rebel-held oil port of Ras Lanuf, in a renewed offensive. Warplanes fired missiles on residential areas and near rebel positions in the desert, witnesses said. There were no reports of casualties. Meanwhile, the rebels said they had been approached by pro-Gaddafi officials offering to hold talks on an exit for the Libyan leader. The rebels rejected the offer, a spokesman said. A Libyan foreign ministry official denied Tripoli had floated a proposal under which leader Gaddafi could step down in exchange for guarantees about his future. The official, an advisor to the deputy foreign minister, said reports about such a deal were absolute nonsense. The Libyan leader had not sent anyone himself, but lawyers from Tripoli had volunteered to act as go-betweens, former Justice Minister Mustafa Abdel Jalil, who heads the rebels Transitional National Council, told AFP. The air strikes have died away because there is low cloud. The warplanes need to see their targets properly and are scared of coming too low because there are a great deal of rebel anti-aircraft guns on the ground. The rebels are firing off the guns all the time, probably to scare off Col Gaddafis troops. In western Libya the opposition-held towns of Zawiyah and Misrata are said to still be under siege from government forces. Forces loyal to Gaddafi launched an artillery and tank attack on Zawiyah in a renewed attempt to capture the rebel-held town. Gaddafis forces had surrounded the strategic town that lies 50 km west of Tripoli near an important oil refinery, the Al Jazeera network said. Meanwhile, western powers are stepping up their efforts to put in place a no-fly zone over Libya. Britain and France are drafting a UN resolution, which will be debated by Nato defence ministers on Thursday. The Organisation of the Islamic Conference has joined the calls for a no-fly zone. Meanwhile, one of Gaddafis sons, Saadi, said on Monday his father had not yet thrown his army into full battle against rebels, saving it to shield Libya against foreign attack, and civil war could erupt if he did.