Syrian combat aircraft and artillery have pounded two key areas of Aleppo, extending the army's campaign to control the country's biggest city.
Heavy gunfire sounded from the Salaheddin district in the southwest of the city on Tuesday, as the opposition denied the government's claims that rebel fighters had been driven out by troops loyal to President Bashar al-Assad.
Instead, rebel fighters said it was the regime troops that had been forced to retreat.
In the latest fighting on Tuesday, attack helicopters turned their machineguns on rebel-held eastern districts for the first time and a MiG warplane later strafed the same area.
The army said two days ago it had taken Salaheddin, but Syrian state television said on Tuesday that government forces were now pursuing the remnants of a group of "terrorists" there, suggesting the army did not have full control of the area.
Salaheddin resembled what a Reuters journalist called a "ghost town", its shops shuttered, with no sign of life in its
apartment buildings and its streets mostly devoid of traffic.
Rebels fired machine guns and assault rifles around street corners at invisible enemies. Wounded civilians and fighters were carried to makeshift dressing stations.
A rebel commander in Aleppo said his fighters' aim was to push towards the city centre, district by district, a goal he believed they could achieve "within days, not weeks".
The rebels now control an arc that covers eastern and southwestern districts.
In-depth coverage of escalating violence across Syria
"The regime has tried for three days to regain Salaheddin, but its attempts have failed and it has suffered heavy losses in human life, weapons and tanks, and it has been forced to withdraw," said Colonel Abdel-Jabbar al-Oqaidi, head of the Joint Military Council, one of several rebel groups in Aleppo.
Salaheddin lies on a major road that the army could use to bring reinforcements into the city.
Hospitals and makeshift clinics in rebel-held eastern neighbourhoods were reportedly filling up with casualties from a week of fighting in the city, a commercial hub only recently drawn into the 16-month-long revolt.
"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 UN observer mission in Syria said the army was using helicopters, tanks and artillery in its assault on the rebels.
It appealed to both sides to protect civilians as UN humanitarian chief Valerie Amos said in a statement that an estimated 200,000 people had fled from Aleppo in two days and an unknown number were still trapped in the city.
Amos said in New York on Sunday that she was "extremely concerned by the impact of shelling and use of tanks and other heavy weapons" on civilians in Aleppo, Damascus and other locations.
She said that many people in Aleppo had sought shelter in schools and other public buildings. "They urgently need food, mattresses and blankets, hygiene supplies and drinking water."
The UNHCR said about 7,000 people had taken refuge in the city's university dormitories and more were camped out in 32 schools, each housing 250-350 people.
Elsewhere in the country, there was renewed fighting on Tuesday in the eastern city of Deir al-Zor, according to the British-based Syrian Observatory for Human Rights, which said more than 300 people had been killed in violence there over the past month.
The group said 70 per cent of the city's residents had fled and estimated that 500,000 of the 1.6 million inhabitants of the surrounding province, which hosts Syria's main oil fields, had been displaced.
Meanwhile, Syrian opposition figure Haitham al-Maleh told reporters that he had been tasked with forming a transitional government in exile based in Cairo.
Radwan Ziadeh reacts to the announcement
of a transitional government in exile
Maleh, 81, is a lawyer and human rights activist who spent several years in prison in Syria.
"We don't want to find ourselves in a political or administrative vacuum" should Assad fall, Maleh said. "This phase calls for co-operation from all sides."
Al-Maleh spoke in a news conference called to unveil the new Council for the Syrian Revolution which he said would act as an alternative to the Syrian National Council (SNC), the main opposition bloc, which he said "had failed to help the Syrian revolution".
The announcement was denounced by the SNC, which has earlier announced plans to launch a government in exile.
Radwan Ziadeh of the SNC told Al Jazeera that the council thinks the formation of any government in exile needs more consultations with other groups, including with the opposition inside Syria.
Ziadeh said al-Maleh is "very credible and well-respected" inside and outside Syria but that his latest move would create "more division" among the opposition.