The U.N. humanitarian chief was heading to Damascus on Wednesday to urge the Syrian government to allow relief workers access to the hardest-hit areas in a country where a year-old uprising has killed thousands of people. Valerie Amos, whose request to visit Syria was rebuffed earlier this month by the government as regime forces attacked a rebel-held area in Homs, was due to arrive in Damascus on Wednesday and leave Friday. The aim of her visit is "to urge all sides to allow unhindered access for humanitarian relief workers so they can evacuate the wounded and deliver essential supplies," she has said. The trip comes as Syrian President Bashar Assad defies mounting international pressure to end the year-old crackdown on an uprising against him. According to state news agency SANA, Assad said Tuesday that he will continue to confront "foreign-backed terrorism." Since the uprising began last March, he has blamed armed gangs and foreign terrorists for the unrest, not protesters seeking change. The U.N. says more than 7,500 people have been killed since Syria's uprising began. Despite the growing bloodshed, President Barack Obama said Tuesday that unilateral U.S. military action against Assad's regime would be a mistake. Russia and China, powerful Syrian allies that have blocked a Security Council resolution against Syria, have made clear they were still standing by the regime in Damascus. Still, in a sign of China's growing alarm, Chinese Commerce Minister Chen Deming said Beijing was pulling its workers out of Syria because of the violence. "I can tell you most Chinese workers have been withdrawn from that country to China," he told a news conference Wednesday. There are only about 100 people left there taking care of projects, assets and property. We will wait until the local situation stabilizes. We will go back to Syria and restart those projects. The top U.S. commander in the Middle East said the advanced air defense weapons Russia has provided to Syria would make it difficult to establish a no-fly zone there as part of an effort to help the rebellion. Marine Gen. James Mattis, head of U.S. Central Command, told the Senate Armed Services Committee it would take a significant military commitment even to create safe havens in Syria where aid could be delivered, as McCain suggested.