Forces loyal to Muammar Gaddafi bombarded Misrata with rockets and artillery on Monday and pounded the insurgents' eastern frontline outpost of Ajdabiyah, rebels said.
A rebel spokesman said 17 people were killed in shelling on Sunday in Misrata, Libya's third-largest city, which has been under heavy bombardment for the past five days, and shelling continued on Monday.
Misrata is the rebels' only major stronghold in the west of the country and has been under siege by pro-Gaddafi forces for the past seven weeks. Evacuees say conditions there are becoming increasingly desperate. Hundreds of civilians are believed to have been killed.
"The Gaddafi forces are shelling Misrata now. They are firing rockets and artillery rounds on the eastern side -- the Nakl el Theqeel (road) and the residential areas around it," Abdubasset Abu Mzeireq said from the coastal city.
He said about 100 people were also wounded in Sunday's clashes, mostly civilians.
Pro-Gaddafi forces also kept up an offensive on the rebels' eastern frontline outpost of Ajdabiyah, which rebels want to use as a staging post to retake the oil port of Brega, 50 miles (80 km) to the west.
One witness said he saw around a dozen rockets land near the western entrance to Ajdabiyah on Sunday and many fled as explosions boomed across the town.
"There are still some guys out there at the western gate but the situation isn't very good," said Wassim el-Agouri, a 25-year-old rebel volunteer waiting at Ajdabiyah's eastern gate.
"We want weapons, modern weapons," said rebel Ayman Aswey, 21. "If we had those, we could advance against them."
Sunday marked a month since the U.N. Security Council passed a resolution authorising force to protect civilians in Libya, leading to an international air campaign.
But despite NATO air strikes against Gaddafi's armour, rebels have been unable to hold gains in weeks of back-and-forth fighting over the coastal towns in eastern Libya.
With NATO troops bogged down in Afghanistan, Western countries have ruled out sending ground troops, a position reinforced by the British prime minister on Sunday.
"What we've said is there is no question of invasion or an occupation -- this is not about Britain putting boots on the ground," David Cameron told Sky News in an interview.
Ajdabiyah's streets were almost deserted on Sunday and rebels barricaded the roads with concrete blocks, tree branches and anything else they could find.
"We are ready for a street war. We are prepared. We have got dynamite and we've got grenades," said rebel fighter Emtar el-Farjany, who was holding a stick of dynamite.
Scores of volunteer fighters and civilian cars carrying men, women and children on Sunday streamed east from Ajdabiyah up the coast road towards Benghazi, where the popular revolt against Gaddafi's 41-year rule began on Feb. 17.
The United States, France and Britain said last week they would not stop bombing Gaddafi's forces until he left power, although when or if that would happen was unclear.
The rebels pushed hundreds of kilometres towards the capital Tripoli in late March after foreign warplanes began bombing Gaddafi's positions to protect civilians, but proved unable to hold territory and were pushed back as far as Ajdabiyah.