Forces loyal to Muammar Gaddafi bombarded the rebel-held city of Misrata on Friday with tanks and heavy artillery, rebels and residents said.
"There have been heavy bombardments since 7 o'clock (0500 GMT) this morning. They are bombing everything, the houses, the centre of the city," insurgent 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 UN 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 UN 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. (Reporting by Mariam Karouny in Tunis and Maria Golovnina in Tripoli, writing by Silvia