In the largest push on Moammar Gaddafi’s home town in weeks, revolutionary fighters on Friday assaulted a convention center in the heart of the city that forces loyal to the ousted leader had turned into their main base.

The forces of Libya’s new rulers advanced into the port city from the west, east and south, attempting to squeeze Gaddafi loyalists into an ever-smaller area during what commanders described as a final offensive to crush resistance in Sirte after a lengthy siege.

The two sides battered each other with rockets, mortar shells and tank fire, as Gaddafi snipers fired on fighters advancing through housing complexes. Smoke drifted over the skyline and explosions thundered throughout the city, as long lines of residents fleeing by car formed at revolutionary forces’ checkpoints.

Fighters entering through the western gate quickly advanced to within a mile of the city center but faced heavy resistance from a loyalist force of roughly 800 men, according to one commander’s estimate.

At least 12 revolutionary fighters were killed and 195 were wounded, doctors said.

Sirte is considered the most crucial of the areas that remain in the hands of Gaddafi’s supporters more than a month after revolutionaries swept into the capital, Tripoli, toppling him from power.

Leaders of the interim government have said that once Sirte falls, they can start a timetable for elections. Sirte is key to the physical unity of the country, since it lies roughly in the center of the coastal plain where most of Libya’s 6.5 million people live, blocking the easiest routes between east and west. Gaddafi loyalists, however, still control another major city, Bani Walid, in the central mountains, and Sabha, deep in the southern desert.

Revolutionary fighters have faced stiff resistance during their siege of Sirte, simultaneously inching their way in and moving to encircle the city before the final assault. Gaddafi’s loyalists have been barricaded in the Ouagadougou Center, a grandiose conference hall that Gaddafi built to host international summits. From there they have been able to defend surrounding residential areas.

Also Friday, a senior U.S. defense official traveling with Defense Secretary Leon E. Panetta said that although Gaddafi’s whereabouts remain unknown, he lacks the ability to marshal resistance to NATO or Libyan opposition forces.

“We have no indication that Gaddafi exercises command and control of militias loyal to him,” said the official, who spoke on the condition of anonymity to discuss intelligence matters.

Panetta visited U.S. and NATO bases in Italy on Friday to receive an update on the Libyan conflict from military commanders.

The defense official also said that Sirte could fall in days or weeks.