Syrian troops say they have recaptured a district in Aleppo and pledge to crush the opposition there. Opposition rebels have insisted that they remain in control.
Syrian troops used helicopter gunships to fire on the district of Salaheddine on Sunday and claimed to be in control of the area by nightfall.
"Complete control of Salaheddine has been [won back] from those mercenary gunmen," an unidentified military officer told Syrian state television late on Sunday. "In a few days safety and security will return to the city of Aleppo."
"The Syrian people are determined not only to confront this plot but also to achieve victory ... They [the rebels] were defeated in Damascus and will be defeated in Aleppo," Syrian Foreign Minister Walid al-Muallem said.
Rebel fighters, meanwhile, claimed to have held off Assad's forces in Salaheddine. The claims could not be independently verified.
In recent weeks, government forces have succeeded in imposing control over the capital, Damascus, but rebel fighters have gained control of parts of Aleppo, Syria's largest city and commercial hub where 2.5 million live.
The battle for Aleppo may determine the success or failure of the uprising in Syria, which got under way in March 2011, and has thus far claimed the lives of 19,000 people, according to the Britain-based Syrian Observatory for Human Rights.
Hard on residents
Most shops in Aleppo were shuttered and closed, with "Strike" painted over the shutters. The only shop doing business was a bakery selling subsidized bread, and the lines for bread stretched around the block.
UN Undersecretary-General for Humanitarian Affairs Valerie Amos said 200,000 people had already fled the fighting in and around the city.
"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," she said.
The violence is making it difficult for humanitarian agencies to reach the victims.
US Defense Secretary Leon Panetta said attacks on the city spelling an end to the Assad regime.
"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," Panetta said, speaking to reporters at the start of a weeklong trip to the Middle East and North Africa.
"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," he said.
"It's no longer a question of whether he's coming to an end, it's when," he added.
The fighting in the city is expected to continue, despite international public outcry, and it is believed that Aleppo will become the decisive test of Assad's ability to quash the 17-month rebellion.
World leaders have condemned the violence, urging both sides to end the bloodshed. Head of the Arab League Nabil al-Arab said the killings in Syria were war crimes. "What is taking place in Syria, especially in the city of Aleppo, is tantamount to war crimes," the Egyptian official Middle East News Agency quoted him as saying.
At least 95 people were reportedly killed across Syria on Sunday.
tm/ccp (AFP, dpa, Reuters)