There has been heavy fighting in the Libyan city of Sirte, where armed supporters of the transitional authorities are facing strong resistance from Gaddafi loyalists.A BBC correspondent on the outskirts of the city says the two sides have been exchanging machine-gun fire, rockets and artillery shells.Many thousands of civilians remain in Sirte, east of the capital, Tripoli.Humanitarian agencies have expressed concern about the conditions they face.Sirte remains one of the final strongholds of supporters of the beleaguered colonel, with Bani Walid 250km (155 miles) to the west the only other major city holding out.Urban snipersHundreds of National Transitional Council (NTC) troops are inside Sirte, but snipers were holding off an advance into the centre of the city, news agency Reuters reported.For a second day, anti-Gaddafi forces were pinned down at a roundabout about 2km (1.5 miles) from the city centre, it said."Gaddafi forces have placed a lot of snipers around the roundabout and it is not easy for us to advance forward until we get rid of the snipers," Ahmed Saleh, an NTC fighter there, told Reuters.The agency said explosions of artillery rounds and exchanges of small arms fire could be heard, and Nato warplanes were flying overhead.There have also been clashes at the port.Humanitarian agencies warn civilians inside Sirte and Bani Walid have appealed for help, saying medical supplies and food are running short.Once the fighting gets underway and both sides establish front lines, there will be a good indication of whether Sirte will fall quickly or whether it will descend into dangerous urban warfare which would kill and injure many civilians and soldiers, says the BBC's Alastair Leithead in the city.The fire power and determination of the new Libyan government's army will take Col Gaddafi's home town, he says, but with propaganda saying the rebels want revenge, those defending Sirte may fight to the death thinking they have nothing to lose.