Attacks by the Assad regime on Syria's largest city Aleppo could be the final "nail in the coffin" for the leader, US Defence Secretary Leon Panetta has warned.
The United Nations says 200,000 civilians have fled Syria's second city, which is its commercial capital, after the regime deployed tanks and helicopter gunships to try to claw back territory lost to rebel fighters.
Many more are believed to be trapped as fierce fighting contnues between
forces loyal to President Bashar al Assad and his opponents.
Speaking at the start of a week-long trip to the Middle East and North Africa, Mr Panetta said the overwhelming force being used by the regime would be its downfall.
"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," he said.
"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'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."
Mr Panetta said Mr Assad had "lost all legitimacy, and the more violence he engages in, the more he makes the case that the regime is coming to an end."
He proclaimed: "It's no longer a question of whether the regime will fall, it's when."
The opposition Syrian National Council (SNC) 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.
More than 20,000 people have been killed, including 14,000 civilians, since the uprising against President Assad's rule erupted in March 2011, according to the British-based Syrian Observatory for Human Rights.
"The United States and the international community have made very clear that this is intolerable, and have brought diplomatic and economic pressure on Syria to stop this kind of violence, to have Assad step down and to transition to a democratic form of government," Mr Panetta added.
Fighting for the past several days has centred on the Salaheddine district in southwest Aleppo, where government troops have been backed by attack helicopters.
Valerie Amos, UN Undersecretary-General for Humanitarian Affairs, said the violence across Syria was making it difficult for relief agencies to reach those trapped by the conflict.
She said 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.
"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.
Peace envoy Kofi Annan urged both sides to hold back, saying only a political solution could bring an end to the conflict.
"The escalation of the military build-up in Aleppo and the surrounding area is further evidence of the need for the international community to come together to persuade the parties that only a political transition, leading to a political settlement, will resolve this crisis," he said.
