Forces loyal to Muammar Gadafy bombarded Misrata with rockets and artillery today and pounded the insurgents' eastern frontline outpost of Ajdabiyah.
Rebels said four civilians were killed in the port city today.
A chartered ship evacuated nearly 1,000 foreign workers and wounded Libyans from the city as rebels claimed they had gained ground in fighting with government forces.
"We wanted to be able to take more people out but it was not possible," said Jeremy Haslam, who led the International Organisation for Migration (IOM) rescue mission.
"Although the exchange of fire subsided while we were boarding ... we had a very limited time to get the migrants and Libyans on board the ship and then leave."
A rebel spokesman said four civilians were killed and five wounded by government shellfire which pounded Misrata, Libya's third-largest city and the rebels' main stronghold in the west of the country, for a fifth day today.
But rebels also said they had gained ground in fighting on bitterly contested Tripoli Street, a key battleground in the city's seven-week siege by forces loyal to President Muammar Gadafy.
"The revolutionaries have made progress and are controlling some areas surrounding Tripoli Street, but danger is still there because (Gadafy's) snipers ... are now using (rocket-propelled grenades) RPGs and bombs," Abdelsalam, a rebel spokesman in Misrata, told Reuters by telephone.
"It is clear Gadafy wants to wipe out Misrata. Nato's inaction is helping him carry out this plan. Are they waiting for a massacre to happen to realize that they need to change tactics?" Abdelsalam said.
"They need to realise that their large aircraft are no longer useful. Gadafy changed tactics. this is urban warfare."
Abdelsalam also said the death toll from yesterday's shelling had risen to 25 as some of those who had been critically wounded had died. He did not say whether that figure included rebel fighters.
Pro-Gadafy 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, 80km to the west.
Tens of thousands of people are trapped in Misrata and other towns following more than a month of fighting which has killed at least 300 and injured more than 1,000, including many children.
Some severely injured people are unable to be evacuated for medical attention, hospitals lack electricity and water and people have been trapped indoors for weeks with little or no food. The United Nations is concerned that consumption of untreated water from wells could lead to outbreaks of water-borne diseases.
Yesterday marked a month since the UN Security Council passed a resolution authorising force to protect civilians in Libya, leading to an international air campaign.
But despite Nato air strikes against Col Gadafy'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 yesterday.
"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.
The United States, France and Britain said last week they would not stop bombing Col Gadafy'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 the Libyan leader's positions to protect civilians, but proved unable to hold territory and were pushed back as far as Ajdabiyah.
Reuters