Syrian combat aircraft and artillery are targeting Aleppo as the army battles for control of the country's biggest city, where rebel fighters say troops loyal to President Bashar al-Assad have been forced to retreat.
Large clouds of black smoke rose into the sky on Tuesday after attack helicopters turned their machine guns on the eastern districts for the first time in the latest fighting and a MiG fighter jet strafed the same area.
After nightfall, video filmed by Reuters news agency showed flashes and loud explosions somewhere in the city area.
At least 10 volleys of shells lit up the night sky and drowned out the sound of the Islamic call to prayer.
Earlier, the outgunned rebels laid hands on tanks and other weapons during an attack on a vital government checkpoint north of Aleppo city, amateur video obtained by Reuters showed.
Rebels are seen in the video with tanks, armoured personnel carriers and other weapons at Anadan, about 20km north of Aleppo.
Battle for Anadan
Lightly armed rebels have been battling for control of Aleppo and the surrounding area for several days against government troops with air power and heavy weapons.
One rebel fighter said the battle to seize Anadan lasted through the night into Tuesday.
Wrecked military vehicles could be seen alongside the intact ones, including a tank with its turret blasted off. A number of Syrian government troops were captured.
While rebels seized two police stations, Syrian ground forces assaulted the opposition strongholds of Salaheddine and Seif al-Dawla in Aleppo's southwest, activists said on Tuesday.
Government helicopters also targeted those neighbourhoods.
Elsewhere, there was renewed fighting on Tuesday in the eastern city of Deir Az-zor, according to the UK-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.
Assad regime criticised
Syria's civil war has entered a far more violent phase since July 18 when a bomb killed four top members of Assad's inner circle.
With the bloodshed mounting, the Arab League chief accused Assad's government of atrocities.
"The massacres that are happening in Aleppo and other places in Syria amount to war crimes that are punishable under international law," Nabil Elaraby, the Arab League secretary-general, said in Cairo on Tuesday.
Separately, in a new report on the Aleppo carnage released on Tuesday night, Amnesty International, the UK-based rights organisation, said.
In-depth coverage of escalating violence across Syria
"Scores of demonstrators and bystanders, most of them young men and boys but including several children and older men, have been shot dead and hundreds injured in the city by security forces and the notorious Shabiha, the armed militias working alongside government forces.
"Some of the victims were bystanders who were not taking part in the demonstrations.
"Families of demonstrators and bystanders shot dead by security forces have been pressured to sign statements saying that their loved ones were killed by 'armed terrorist gangs'."
Life for Aleppo's three million residents is becoming increasingly unbearable under the military siege.
"The humanitarian situation here is very bad,'' Mohammed Saeed, an activist living in the city, told the Associated Press news agency by Skype.
"There is not enough food and people are trying to leave. We really need support from the outside. There is random shelling against civilians.
"The city has pretty much run out of cooking gas, so people are cooking on open flames or with electricity, which cuts out a lot."
Days of shelling
Days of shelling have forced many civilians to flee to other neighbourhoods or even escape Aleppo altogether.
The UN said on Sunday that up to 200,000 people had left the city.
The UN observer mission in Syria has accused the army of using helicopters, tanks and artillery in its assault on the rebels.
It has appealed to both sides to protect civilians.
Valerie Amos, UN humanitarian chief, 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.
She 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.
Amos 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.
On the political front, a group of exiled Syrian activists announced a new opposition alliance on Tuesday that aims to form a transitional government - a challenge to the Syrian National Council (SNC), a longer established group that they said had failed.
The launch of the so-called Council for the Syrian Revolution marks the latest effort by Syria's divided opposition to forge a political alternative to Assad whose forces are trying to put down a 16-month armed uprising.
"The brothers have asked me to form a transitional government in Syria and to begin dialogue with the rest of the Syrian opposition," Haitham al-Maleh, a former judge, told a Cairo news conference called to unveil the new body.
Maleh told Reuters the new alliance would act as an alternative to the SNC which he said "had failed to help the Syrian revolution". It would work to get more help to rebels, he said.
While the SNC has been an international voice for the opposition, activists on the ground have complained that the exiled leadership has little connection to what is happening in Syria.
Maleh, a long-standing dissident against the Assad family's four decades in power, resigned from the SNC in March, saying he had given up trying to make the group more effective.
"We are not asking for military intervention, such as an invasion, but international protection, such as stopping Syrian planes," he said.
"When Aleppo is freed, we will have the northern part of Syria and will ask [the opposition] to return home."
The Council for the Syrian Revolution comprises 70 opposition figures and will be based in Cairo, with branches in Lebanon, Turkey, Jordan and Iraq.
Reacting to the council's formation, Abdelbasset Seida, SNC's head, said all opposition figures were free to work in the way they thought would help the Syrian revolution.
"But the process of forming a transitional government is difficult and needs consultations with all members of the Syrian opposition, rebels and the Free Syrian Army," Seida told Reuters.
"But if each group came out alone announcing a formation of a new government without talks and research, this would end up in having a series of weak governments that don't represent anyone and would not be able to mean or do anything," he said.
"Our end goal is to form a government that would represent and please all members of the Syrian community."
Radwan Ziadeh of the SNC told Al Jazeera the SNC thinks the formation of any government in exile needs more consultations with other groups, including with the opposition inside Syria.
He said Maleh is "very credible and well-respected" inside and outside Syria but that his latest move would create "more division" among the opposition.