The offensive began Saturday and killed at least 13 people, including a 7-year-old boy, Britain's Sky News reported. It escalated Sunday, a rebel identified only as Ibrahim told the broadcaster.
He said reinforcements to the government forces were arriving and snipers had taken up positions on rooftops.
There was no immediate casualty toll given for Sunday's fighting.
Rebels had seized control of the town in February when the uprising began, but government forces regained control of it in March.
Elsewhere, al-Jazeera reported another fierce battle between forces loyal to dictator Moammar Gadhafi and rebels between the towns of Zintan and Yafran in the west.
The Arabic broadcaster said rebels had pinned down army positions, but army reinforcements and heavy artillery was preventing further rebel gains.
In late March, the United Nations Security Council authorized NATO to protect civilians from Gadhafi's military.
Sunday, British Foreign Secretary William Hague told Sky News Gadhafi's resistance to stop the fighting and step down was wearing down.
"Time is on our (NATO's) side, not on his side," Hague said. "The position of the Gadhafi regime is weakening all the time."