The Syrian military has stepped up its campaign to drive rebel fighters out of Aleppo, but rebels have said they were still holding firm in the country's biggest city.
So far, 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 later driven out.
Opposition activists denied a government declaration that its forces had recaptured the Salaheddine district, in southwest Aleppo.
It straddles the most obvious route for Syrian troop reinforcements coming from the south.
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 that had previously stayed out of a 16-month-old 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.
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 16 months into the uprising.
An unidentified Syrian army officer said on state television late last night that troops had pushed "those mercenary gunmen" completely out of Salaheddine, adding: "In a few days, safety and security will return to the city of Aleppo."
Reuters journalists in Aleppo have been unable to approach Salaheddine to verify who controls 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.
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 subsidised 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.
Rebel fighters remain in control of swathes of the city, moving around those areas armed with assault rifles and dressed in items of camouflage clothing in an edgy show of confidence.
They were emboldened to strike at Aleppo and central Damascus by a 18 July explosion that killed four of Assad's top security officials.
The rebels include small numbers of foreign fighters drawn from other Arab countries, commanders in northwest Syria say. Some rebel checkpoints in Aleppo were flying black and white banners of Islamist militants.
US Defence Secretary Leon Panetta said attacks on Aleppo were putting the nail in the coffin of Mr Assad's government, showing he lacks the legitimacy to rule.
"If they continue this kind of tragic attack on their own people in Aleppo, I think it ultimately will be a nail in Assad's own coffin," Mr Panetta said.
"What Assad has been doing to his own people and what he continues to do to his own people makes clear that his regime is coming to an end. It's lost all legitimacy.
"It's no longer a question of whether he's coming to an end, it's when."
Turkish military convoy heads to border
Turkey has sent a convoy of about 20 vehicles carrying troops, missile batteries and armoured vehicles to the border with Syria.
It is the latest in a series of deployments in the region in recent weeks.
There has been no indication that Turkish forces will cross the border and the troop movements may be strictly precautionary in the face of the increasing violence in Syria.
The convoy left a base in Gaziantep province to head south to Kilis province, where the troops will stay, the state-run Anatolian news agency said.
Witnesses said the troops and vehicles have left a major highway and are now stationed along a fenced-off section on the frontier with Syria.
Meanwhile, UN Undersecretary-General for Humanitarian Affairs Valerie Amos has said 200,000 people have fled the fighting in and around Aleppo in the last two days.
Ms Amos said that the violence across Syria has made it hard for humanitarian agencies to reach people in need.
She said: "Many people have sought temporary shelter in schools and other public buildings in safer areas.
"They urgently need food, mattresses and blankets, hygiene supplies and drinking water."