Around 200,000 civilians have fled fighting in Syria's most populous city Aleppo and many more were trapped, the UN said as a fierce government offensive against rebels entered a second day.
The opposition Syrian National Council (SNC) on Sunday accused the government of preparing to carry out "massacres" in the northern city and pleaded for heavy weapons to enable rebels to meet the regime onslaught.
The SNC also urged the UN to hold an emergency session to discuss ways to protect civilians caught up in the conflict.
Syrian Foreign Minister Walid al-Muallem, on a surprise visit to key ally Iran, said the rebels "will definitely be defeated" in Aleppo, even as a Free Syrian Army (FSA) commander boasted the city would become a "graveyard" for the army's tanks.
The Syrian Observatory for Human Rights said Sunday's fighting was focused around the southwestern neighbourhood of Salaheddin, where rebels repulsed a ground assault on Saturday.
"There are clashes on the edges of... Salaheddin" which regime forces were pounding with helicopter gunships, the Observatory's Rami Abdel Rahman told AFP, describing the situation as "a full-scale street war".
After massing for two days, troops backed by tanks and helicopters on Saturday launched a ground assault on Salaheddin, where rebels concentrated their forces when they seized much of Aleppo on July 20.
Both sides claimed to have made advances, but an AFP correspondent reported rebels had largely repulsed the army when it launched its first onslaught.
Civilians in the city of some 2.5 million crowded into basements seeking refuge from the intense bombardment by artillery and helicopters, the correspondent said.
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 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," she said.
Colonel Abdel Jabbar al-Oqaidi, FSA commander for Aleppo, said the rebels had inflicted heavy losses on the army in Salaheddin but that there had been many civilian deaths.
"Aleppo will be the graveyard of the tanks of the Syrian army," Oqaidi told AFP in an interview at an isolated farmhouse surrounded by olive groves near the city.
"We ask the West for a no-fly zone" in order to prevent aerial operations by Assad's forces, he said.
The colonel said his men were positioned across Aleppo and would not withdraw as they had when they came under intense fire from regime troops in Damascus earlier this month.
"There is no strategic withdrawal of the Free Syrian Army. We await the attack," he said, while refusing to reveal how many rebels were fighting in Aleppo.
"We expect (the army) to commit a very great slaughter, and we urge the international community to intervene to prevent these crimes," he said.
The British-based Observatory reported that 67 people were killed across the country on Sunday.
US Defense Secretary Leon Panetta warned Syrian President Bashar al-Assad that the assault on his own population in Aleppo would be a nail in his coffin.
"It's pretty clear that Aleppo is another tragic example of the kind of indiscriminate violence that the Assad regime has committed against its own people," Panetta told reporters on a military plane en route to Tunisia.
"And in many ways, if they continue this kind of tragic attack on their own people in Aleppo, I think ultimately it will be a nail in Assad's coffin," he said.
"He's just assuring that the Assad regime will come to an end by virtue of the kind of violence they're committing against their own people."
In Tehran, Muallem vowed regime forces would crush the rebels in Aleppo.
"We believe that all the anti-Syrian forces have gathered in Aleppo to fight the government... and they will definitely be defeated," he told a joint news conference with Tehran's Foreign Minister Ali Akbar Salehi.
As the rebels faced the superior firepower of Assad's regime, SNC chief Abdel Basset Sayda called on foreign governments to provide them with heavy weapons.
"We want weapons that would stop tanks and jet fighters," Sayda said after talks in Abu Dhabi.
The SNC also called on the Security Council to hold an emergency session on the situation in Aleppo, Damascus and Homs, urging it to "take action to provide civilians with the needed protection from brutal bombing campaigns".
Peace envoy Kofi Annan urged both sides to hold back, saying only a political solution could end a conflict that rights activists say has killed more than 20,000 people since the uprising erupted in March 2011.