Forces of the Libyan transitional government have launched a new offensive against the pro-Gaddafi stronghold of Sirte after a two-day lull in fighting.Barrages of rocket and tank fire have been directed at the town centre.Hundreds more civilians have escaped from the town, but humanitarian workers fear there are thousands more unable or unwilling to leave.In the south, over 1,200 African migrants are being evacuated to Chad.'Final assault'The transitional government's forces have brought in most of their artillery to try to dislodge the loyalists who have been resisting them for the past three weeks, but they are still finding it difficult to advance, says the BBC's Jonathan Head who is on a government front line outside Sirte.A commander on the front line told the BBC it was the start of the final assault on Sirte, but he had no idea how long it would take.Hundreds of the town's residents have left on Monday in overcrowded cars, but they said thousands more remained in Sirte, some injured, some without cars or petrol, some just too frightened to leave their homes.The besieged town is the birthplace of ousted leader Muammar Gaddafi, whose whereabouts remain unknown.Transitional authority forces had observed a truce until Sunday to encourage the remaining civilians to get out, before launching a final assault.Sirte is one of two major cities still holding out against the National Transitional Council (NTC) forces.Sabha evacuationIn southern Libya, the International Organization for Migration (IOM) is evacuating 1,200 African migrants from the southern Libyan city of Sabha, which was captured from pro-Gaddafi forces last month.A convoy of 15 trucks has left for Niger on the way to Chad, from where they will be transported home.IOM chief of mission in Chad Qasim Sufi has said the group of people from 11 different nations are "extremely relieved" after being trapped at a transit centre in the embattled city.Fighting has prevented the IOM from getting the group out of the centre or bringing supplies to them in Sabha.Many Africans complained they were suspected of being mercenaries, and treated with considerable hostility by forces of the National Transitional Council.