The Syrian military has stepped up its campaign to drive rebels out of Aleppo, where rebel fighters said they were holding firm.
The rebels also vowed to turn the country's largest city into the “regime's grave''.
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.
A Reuters reporter heard helicopters firing heavy machine-guns over the eastern part of the city on Tuesday for the first time in several days.
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 to 40 people, and this does not include the dead bodies,''said a young medic in one clinic.
“A few days ago we got 30 injured persons and maybe 20 corpses, but half of those bodies were ripped to pieces, because we can't figure out who they are.''
The opposition Syrian Observatory for Human Rights said more than 100 people, 73 of them civilians, were killed in Syria on Monday.
It said five rebel fighters died during clashes with Syrian forces in Salaheddine.
Outgunned rebel fighters, patrolling in flat-bed trucks flying green-white-and-black “independence'' flags, said they were holding out in Salaheddine in spite of a battering by the army's heavy weapons and helicopter gun-ships.
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 Aleppo, where defeat would be a serious strategic and psychological blow.
Military experts say 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.''(Reuters/NAN)