Forces loyal to Muammar Gaddafi have continued their heavy bombardment of residential areas of Misrata amid mounting concern that the army has fired cluster bombs into the besieged rebel stronghold.

At least 100 Soviet-designed Grad rocket rockets were fired into the eastern city today, a rebel spokesman said. "They fired Grads at an industrial area this morning, at least one hundred rockets were fired. No casualties are reported," Abdelbasset Abu Mzereiq told Reuters.

The Grad, which launches multiple rockets from mobile launchers, has been blamed for a number of civilian deaths in recent days. More than 100 of the rockets landed in the city yesterday as pro-Gaddafi forces reached the city centre, the rebels said.

"Witnesses said they saw pro-Gaddafi soldiers on foot in the city centre today. Except for snipers, they usually stay in their tanks and armoured vehicles," the rebel spokesman added.

The intensifying bombardment came as Human Rights Watch reported that four cluster bombs exploded in the city yesterday and on Thursday, and two Libyan residents of Misrata told the Guardian that they suspected the munitions, banned in most countries, were being used. Cluster bombs explode in midair, indiscriminately throwing out dozens of high-explosive bomblets which cause damage and injuries over a large area. The submunitions often fail to explode on impact but detonate when stepped on or picked up.

Pro-Gaddafi authorities said a Red Cross team had arrived in Misrata to assess the situation at the invitation of the government. "The Libyan army took them to a specific place into the city and the Red Cross went to the other side [the one controlled by the opposition]," said government spokesman Mussa Ibrahim.

The team would issue a report on its preliminary findings, he added. The team was invited before the use of cluster bombs was reported.

The worsening seige of the rebel stronghold follows a commitment by the leaders of US, Britain and France to pursue military action until Colonel Gaddafi has been removed from power. In a joint letter yesterday, Barack Obama, David Cameron and Nicolas Sarkozy described the onslaught on Misrata as a "medieval siege … to strangle its population into submission".

The former head of the UK's armed forces, Lord Dannatt, urged the international coalition to seek a fresh UN security council resolution specifically authorising the training and arming of the rebels, warning that a stalemate would create a vacuum likely to be filled by Islamist extremists.

"We want to act within the law, within international agreement and therefore we should be arguing the case to not accept a stalemate, not to put our own boots on the ground, but to properly arm those boots that are on the ground.

"They are Libyan boots. Let the Libyan people have the wherewithal to choose a new government for themselves," he told BBC Radio 4's Today programme.

"We have got to move this one on, we have got to be innovative about the way we do it. I have thought about it long and hard: go back to New York, get a strengthened UN security council resolution and arm, equip and train the opposition."

The use of cluster bombs had further weakened Gaddafi's position, Dannatt added. "If we thought that Gaddafi had lost the moral right to rule this country a month ago, he has lost it in the last 24 hours, that's for sure."