Thousands of troops loyal to Libyan leader Muammar Gaddafi advanced on the rebel-held western city of Misrata on Wednesday, shelling it from three sides in attacks that killed at least 12 rebels, a rebel spokesman said.
There was no immediate comment from Gaddafi's government.
"Misrata is under heavy shelling ... Gaddafi forces are shelling Misrata from three sides: east, west and south," rebel spokesman Hassan al-Misrati told Reuters from inside the town.
"He has sent thousands of troops from all sides and they are trying to enter the city. They are still outside, though."
He added that 12 rebels had been killed and 26 wounded.
The offensive followed a lull in NATO bombing of Tripoli on Wednesday, after 24 hours of some of the heaviest bombardments of the Libyan capital since air strikes began in March.
Gaddafi's troops and the rebels have been deadlocked for weeks, with neither side able to hold territory on a road between Ajdabiyah in the east, which Gaddafi's forces shelled on Monday, and the Gaddafi-held oil town of Brega further west.
Rebels control the east of Libya, the western city of Misrata and the range of western mountains near the border with Tunisia. They have been unable to advance on the capital against Gaddafi's better-equipped forces.