Libyan government troops are clashing with fighters loyal to ousted leader Col Muammar Gaddafi in one of his last strongholds, Sirte, reports say.Black smoke was seen rising from the coastal city after National Transitional Council (NTC) soldiers reportedly entered from the west.A BBC correspondent says NTC forces have also moved from the east towards Sirte, Col Gaddafi's birthplace.It is not known if Col Gaddafi or his family members are in the city.Sirte, one of two remaining bastions of support for Col Gaddafi, has fiercely resisted attempts by anti-Gaddafi forces to capture it over recent weeks.The BBC's Alastair Leithead, who is with NTC forces about 30km (18.5 miles) to the east, says they have made rapid progress and are exchanging rocket-fire with Gaddafi loyalists defending the city.