Syrian opposition groups dispute number of dead but residents claim new type of artillery shell is being used against restive city

Syrian forces persisted with one of the deadliest attacks of the 11-month uprising on Sunday, pounding parts of Homs even as residents combed through rubble looking for victims of a sustained barrage over the weekend that killed dozens, if not scores of people.

Homs residents said the flashpoint area of Bab al-Amr was under sustained attack throughout the afternoon, with up to six people killed. There was also a renewed bombardment of the nearby neighbourhood of Khalidiyeh, centre on Friday night of one of the most violent assaults thus far by Bashar al-Assad's forces.

Residents were still trying to come to terms with the savage firepower aimed at their district during a six-hour barrage early on Saturday.

"We counted 300 explosions, at least, in that time," said a Bab al-Amr resident, who calls himself Omar Shakir. "There was nothing we could do for the people there, there are two regime checkpoints between us and them. And today it was our turn."

Mortars fired from an elevated area around a mile and a half away accounted for almost all of Friday nights casualties. The numbers of dead are disputed. A hospital in Homs and two Syrian opposition groups say 250-300 may have died. A third group, the Local Co-ordination Council said the toll may be 50-60. Either way, with many civilians caught up in the onslaught, including women and children, it was an offensive that marked a grim low point in the confrontation between Assad's forces and the protesters and armed deserters now ranged against him.

Residents said a new weapon was being used – possibly artillery shells, which have a distinctive whine as they approach their target. "They are very unusual sounds, the explosions," said Shakir.

"We have not heard them before in Bab al-Amr. And we have heard a lot of explosions here."

A Khalidiyeh resident, Fadi, said local people had been able to leave their homes to bury the dead and look for survivors.

"There was random shooting into civilian areas. It was indiscriminate and it went on for six hours. The smell in the streets is completely awful," he said. "The blood is everywhere and it is difficult to find all the parts of the people."

Videos posted on the internet purported to show men digging makeshift graves in which to bury unclaimed body parts.

"We were not expecting that the Syrian army would attack in this way," said Fadi of the mounting horror as Friday night's bombardment intensified. "We thought it was some kind of exaggeration that would be over soon, but the explosions kept coming and coming."

By daybreak, the small neighbourhood, which had been closed off to regime forces by the Free Syrian Army lay in partial ruins, with the roofs of numerous buildings pancaked onto the floors below. Dozens of people are still believed to be trapped, with residents having no equipment other than shovels with which to rescue them.

A hospital in a rebel-held area of Homs was also reported to have been hit with at least one mortar round , causing extensive damage to part of a ward.

The Syrian government has denied bombarding Homs and says "terrorist gangs" are responsible for the deaths. Syrian officials say some of the bodies had been kidnapped by opposition elements who had killed them in an attempt to persuade Syria's allies, Russia and China, to move against them in the UN security council.

"They are stupid to suggest that we have mortars like that," said the brother of one man linked to the Free Syria Army, which is comprised mainly of defectors carrying light weapons they used while serving in the regular military.

"Anyone who knows anything about military operations knows that mortars are fired from a fixed position (and are) easy to track by radar, or even through lenses. They can be destroyed by artillery, or helicopters within minutes. This went on for six hours."