Libya revolutionary sources said Muammar Gaddafi's forces fired rockets at the western towns of Zintan and Misrata on Friday, Al Arabiya reported.
It was unclear whether the rockets were fired after the government declared a ceasefire in its offensive to crush the Libyan revolt after the U.N. Security Council passed a resolution authorizing military action against Gaddafi's forces.
"Gaddafi's forces are bombing the city with artillery shells and tanks. We now have 25 people dead at the hospital, including several little girls," Dr Khaled Abou Selha told Reuters by satellite phone.
"They are even bombing ambulances. I saw one little girl with half of her head blown off," he said, crying.
The doctor and another resident, Mohammed, said the city was still being heavily shelled despite a rebel claim that the attack had been defeated and the announcement at around 1230 GMT by the foreign minister of a ceasefire.
"There are 20 tanks in the city, they are killing everybody because they want to recapture the city by this evening," Mohamed said. The sound of heavy artillery could be heard in the background.
A rebel fighter had earlier said the insurgents had beaten back the attack, despite the heavy weapons used by Gaddafi's forces and the fact that the city of 300,000, the last big rebel stronghold in western Libya, has been under siege for days.
"There have been heavy bombardments since 7 o'clock (0500 GMT) this morning. They are bombing everything, the houses, the center of the city," opposition fighter Saadoun told Reuters by phone from the city, the last big rebel stronghold in western Libya.
"It's the heaviest bombardment I have seen so far." The sound of heavy artillery could be heard in the background.
The attack began hours after the U.N. Security Council passed a resolution endorsing a no-fly zone and military attacks on Gaddafi's forces to protect civilians.
"We believe they (Gaddafi's forces) want to enter the city at any cost before the international community starts implementing the U.N. resolution," said Saadoun.
"On behalf of all the people of Misrata, the women, the children and the elderly, we call on the international community to do something before it's too late. They must act now," he said. "They already failed us before and were late in taking a decision, they should not repeat the same mistake."
Another insurgent who only gave his name as Mohammed said government tanks were advancing towards the centre of the city, and the rebels were trying to resist them.
"All the people of Misrata are desperately trying to defend the city," he said. Al Arabiya television said an unspecified number of people were killed and wounded and several mosques, schools and residential buildings badly damaged in the attack on the city of 300,000, about 200 km (130 miles) east of Tripoli.
Misrata has been under siege by Gaddafi's forces for days.
"This morning I spoke to a doctor in Misrata, there is heavy bombardment there, explosions inside the city," said Tariq, a doctor from Misrata who now lives in Britain. He said he spoke by phone to colleagues and relatives there on Friday morning.
"They cannot send out any ambulances. They think it's artillery and tanks, shelling, not air strikes. It's very worrying," he said.
Speaking in Tripoli on Wednesday, a government spokesman had said the military operation in the city should be over by Friday morning.
Protecting civilians only
The Arab League chief, meanwhile, said a U.N. Security Council resolution was aimed at protecting civilians and not did not back any invasion, and said he did not want any side "to go too far".
Amr Moussa said participation of any Arab state in implementing a "no-fly" zone or in other action would be discussed on a bilateral level as the League's own resolution backing a "no-fly" zone had not specified other steps by states.
"The goal is to protect civilians first of all, and not to invade or occupy. The resolution is clear on that point," he told Reuters.
"We don't want any side to go too far, including Libya by attacking the civilian population. Our main task is to protect the Libyan civilian population. This is our task, this is our goal," he said.