Damascus - U.S. Defense Secretary Leon Panetta has said that attacks on the Syrian city of Aleppo will be the proverbial nail in the coffin of President Bashar Assad's government. Panetta pointed out that Syrian crisis was deepening and that Assad was hastening his own demise. "If they continue this kind of tragic attack on their own people... I think it ultimately will be a nail in Assad's coffin," the BBC quoted Panetta, as saying. "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. Panetta said he aimed to reinforce an international consensus that Assad must step down and allow a peaceful transition to democracy. Meanwhile, heavy fighting is continuing in Syria's largest city where government forces are trying to oust rebel fighters. UN humanitarian chief Valerie Amos said 200,000 people had fled the fighting in Aleppo and that an unknown number were trapped.