Syrian rebels held off an offensive by regime forces in Aleppo as the head of the exiled opposition called for heavy weapons and said President Bashar al-Assad should be tried for "massacres".
After massing for two days, troops backed by tanks and helicopter gunships moved on southwestern Aleppo where rebels concentrated their forces when they seized much of the northern city on July 20.
At least 29 people were killed, the Syrian Observatory for Human Rights said, contributing to a figure of around 140 nationwide, amid growing concern about the risks of reprisals against civilians in the country's commercial capital.
The London-based watchdog said more than 20,000 people, mostly civilians, have now died since the uprising against Assad's regime erupted in March 2011.
Civilians crowded into basements seeking refuge from the bombing, with the Observatory's Rami Abdel Rahman describing the clashes as the "fiercest" of the uprising.
"There are thousands of people in the streets fleeing the bombardment. They're being terrorised by helicopter gunships flying at low altitude," said an activist, adding many had taken refuge in public parks.
Colonel Abdel Jabbar al-Oqaidi of the rebel Free Syrian Army said his forces had repulsed troops in Salaheddin district and that the regime offensive had been stopped.
"We managed to force the army to the neighbourhood of Hamdaniyeh," he told AFP on the phone, adding that while the army had been halted on the ground, artillery and gunships continued to pound the city.
Abdel Rahman said the fact the soldiers had been stopped in Salaheddin "does not necessarily mean a withdrawal as their strategy is to bombard ... to cause an exodus then launch an assault even more fierce".
An AFP correspondent said rebels were poised to launch a final raid on a strategic police post in the city centre, where 100 men armed with Kalashnikovs have been holding out for three days.
Its capture would open a corridor between Salaheddin and the rebel-held district of Sakhur, some six kilometres (four miles) to the northeast.
Meanwhile Syrian National Council chief Abdel Basset Sayda called for the rebels to be armed and said the opposition would discuss a proposed transitional government with rebel forces.
He insisted that Assad should be tried for "massacres" and not be offered asylum in any future solution.
"We want weapons that would stop tanks and jet fighters. That is what we want," he said following talks in Abu Dhabi.
He urged Arab "brothers and friends to support the Free (Syrian) Army" saying the support should be "qualitative because the rebels are fighting with old weapons".
He said the rebellion needed support in order to achieve a "significant change" in the uprising that began in March last year.
Sayda said the opposition needed a minimum of $145 million monthly to provide basic needs but had received only $15 million over several months.
He added that any future plan should not include an amnesty for Assad, saying he should be put on trial.
"The Yemeni example can't be applied in Syria," he said, referring to the amnesty given to former Yemeni president Ali Abdullah Saleh following mass protests.
"There are massacres being committed. We believe Bashar al-Assad should be tried. He is a criminal and should not be given a shelter," he added.
Pro-government daily Al-Watan had warned the "mother of all battles" loomed in Aleppo as the government moved to reassert its authority after recapturing rebel-held districts of Damascus.
Both sides acknowledged casualties were likely to be high, and world powers have called for "maximum pressure" to stop the regime from carrying out the assault and to avert another massacre.
In late May, at least 108 people were killed near the central town of Houla, the United Nations said. On July 12, regime forces killed more than 150 people in the central village of Treimsa, the Observatory said.
Russia warned a "tragedy" was looming but said it was unrealistic to expect the government would stand by when rebels were occupying major cities.
"We are persuading the government that they need to make some first gestures," said Foreign Minister Sergei Lavrov.
"But when the armed opposition are occupying cities like Aleppo, where yet another tragedy is brewing as I understand ... it is not realistic to expect that they will accept this."
Lavrov accused the West of aiding opposition fighters.
"Our Western partners ... together with some of Syria's neighbours are essentially encouraging, supporting and directing an armed struggle against the regime."
"The price of this is yet more blood," he said.
French President Francois Hollande specifically addressed Russia and China as he urged the UN Security Council to rapidly intervene to pre-empt an all-out civil war in Syria.
Hollande said Assad's regime was doomed and "will therefore use force right to the end".
"The only solution which will allow Syrians to reconcile and reunite is the departure of Bashar al-Assad and the formation of a transitional government," he told reporters.
Turkey warned it could "not remain an observer" as violence raged across its southern border.
"We must do what we can together in the United Nations Security Council, and also in the Organisation of Islamic Cooperation and the Arab League, to make sure that we can make some important progress in trying to avert this appalling situation," said Prime Minister Recep Tayyip Erdogan.