National Transitional Council troops began an assault on Sirte, Libya, one of the last strongholds of former leader Moammar Gadhafi, NTC officials said. Hundreds of vehicles advanced on Sirte, Gadhafi\'s hometown, from Misurata, which is to the east, and from Benghazi, to the west, and reportedly were closing in on the town\'s center, the BBC reported Friday. The NTC gave Sirte residents time to leave before the assault began. Thousands of civilians have fled Sirte but many more are thought to have stayed, the BBC reported. Sustained tank and mortar fire rained on Sirte, destroying buildings and shooting columns of smoke into the air, witnesses said. The Ouagadougou conference center, where NTC officials said they believe many of the forces loyal to Gadhafi have gathered, is among the targets. On Thursday, the NTC military commander, Col. Abdel Salam Gadallah, said 75 percent of the city was under NTC control. \"God willing, in two days maximum all of Sirte will be clean,\" Gadallah said. Efforts to negotiate a surrender failed. On Thursday, Gadhafi urged Libyans to take to the streets as a show of resistance to the interim leaders. Gadhafi, speaking in an audio message broadcast by Syrian-based Arrai television, said conditions in Libya were \"unbearable\" and the NTC was not the country\'s legitimate leader. \"I say to them, do not fear anyone. You are the people; you belong to this land,\" said Gadhafi, whose whereabouts are unknown. Several members of his family are in hiding or fled the country.