President Bashar al-Assad

The Syrian military has stepped up its campaign to drive rebels out of Aleppo, where fighters said they were holding firm, vowing to turn the country's largest city into the "grave of the regime".

Opposition activists denied a government declaration that its forces had recaptured the Salaheddine district, in southwest Aleppo, straddling the most obvious route for Syrian troop reinforcements coming from the south, reports Reuters.

Hospitals and makeshift clinics in rebel-held eastern neighbourhoods were filling up with casualties from a week of fighting in the city, a commercial hub drawn into the 16-month-long revolt against President Bashar al-Assad.

"Some days we get around 30, 40 people, not including the bodies," said a young medic in one clinic. "A few days ago we got 30 injured and maybe 20 corpses, but half of those bodies were ripped to pieces. We can't figure out who they are."

The opposition Syrian Observatory for Human Rights said 40 people, 30 of them civilians, were killed in Syria on Monday. Two rebel fighters died in Salaheddine.

Outgunned rebel fighters, patrolling in flat-bed trucks flying green-white-and-black "independence" flags, said they were holding out in Salaheddine despite a battering by the army's heavy weapons and helicopter gunships.

A fighter jet flew overhead, a reminder of the overwhelming military advantage still enjoyed by government forces.

"We always knew the regime's grave would be Aleppo," said Mohammed, a young fighter, fingering the bullets in his tattered brown ammunition vest.

"Damascus is the capital, but here we have a fourth of the country's population and the entire force of its economy. Bashar's forces will be buried here."

So far, however, the government's superiority on the ground means rebels have had little success in holding on to urban territory. The rebels made a major push into Damascus two weeks ago, but were driven out.

The Syrian government has said it has recaptured Salaheddine. Reuters’ journalists in Aleppo have not been able to reach the neighbourhood to verify who holds it.

The army's assault on Salaheddine echoed its tactics in Damascus earlier this month when it used its overwhelming firepower to mop up rebel fighters district by district.

Assad's forces are determined not to let go of Aleppo, where defeat would be a serious strategic and psychological blow.

Military experts believe the rebels are too lightly armed and poorly commanded to overcome the army, whose artillery pounds the city at will and whose gunships control the skies.

"Yesterday they were shelling the area at a rate of two shells a minute. We couldn't move at all," a man calling himself a spokesman for the "Aleppo Revolution" said on Monday. "It's not true at all that the regime's forces are in Salaheddine."

Warfare has stilled the usual commercial bustle in this city of 2.5 million. Vegetable markets are open but few people are buying. Instead, crowds of sweating men and women wait nearly three hours to buy limited amounts of heavily subsidized bread.

In a city where loyalties have been divided, with sections of the population in favour of the Assad government, some seemed wary of speaking out in the presence of the fighters, many of whom have been drafted in from surrounding areas.