Libya conflict: New offensive on Sirte
Forces of the Libyan transitional government have launched a new offensive against the pro-Gaddafi stronghold of Sirte.
Barrages of rocket and tank fire were being directed at the city centre after a two-day lull in fighting.
Hundreds more civilians have escaped from the town, but humanitarian workers fear there are thousands more unable or unwilling to leave.
In the south, more than 1,200 African migrants are being evacuated to Chad.
National Transitional Council (NTC) forces have brought in most of their artillery to try to dislodge the loyalists who have been resisting them for the past three weeks.
However, 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 there told the BBC it was the start of the final assault on Sirte, but he had no idea how long it would take.
At the scene
They have brought up most of their artillery, a whole array of guns have been brought in, and tank rounds are going out. They have been firing them regularly. So there has been a reasonably sustained artillery barrage going in the direction of Sirte which is about 5km (3 miles) away from here.
But they have not advanced, and it is not clear whether this is what they believe is the final assault, or whether they are simply taking up the fight again, having had this two-day ceasefire. Certainly there is far more artillery than we have heard in the last four days, far more action going on.
But there is no sign the government fighters have a plan to break through. When they get to the centre of Sirte, what they will face is fighting from building to building, and opponents who have proved to be very accurate with their weapons and very determined in their resistance. So, although they are talking about this being a final assault, at this stage it is too early to judge whether what we have seen actually amounts to that.
Hundreds of the town's residents left on Monday in overcrowded cars.
Witnesses 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.
NTC forces had observed a truce until Sunday to encourage the remaining civilians to get out, before launching this new offensive.
Sirte is one of two major cities still holding out against the NTC.
A Nato official has denied a report on a pro-Gaddafi TV station that a Nato helicopter was shot down over Sirte.Sabha evacuation
In 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.
The IOM chief of mission in Chad, Qasim Sufi, has said the group of people from 11 different nations were "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 NTC forces.