BEIRUT - Syrian forces bombarded Homs on Monday, killing 50 people in a sustained assault on several districts of the city which has become a center of armed opposition to President Bashar Assad, the Syrian National Council opposition group said.The bombardment came a day after the United States promised harsher sanctions against Damascus in response to Russian and Chinese vetoes of a draft UN resolution that would have backed an Arab plan urging Assad to step aside."This is the most violent bombardment in recent days," said one activist in Syria who was in touch with Homs residents. Another activist said forces loyal to Assad were using multiple rocket launchers in the attack. According to a report by Al Arabiya, Assad's forces employed military helicopters in their shelling of the city's Baba Amro district, causing the collapse of a number of buildings. The Arab daily quoted an activist as saying that "seven residential buildings collapsed as a result of the Syrian intensive shelling of Homs," adding that the city "is witnessing a real war."Arab satellite television stations broadcast live footage from Homs. Explosions could be heard and smoke was seen rising from some buildings.During the attack, an explosion ripped through an oil pipeline feeding a main refinery in the city of Homs and a plume of smoke was seen rising from the site, residents and activists said.The explosion, the second in a week that hit the pipeline, which carries crude oil from the eastern Rumailan field, occurred in the district of Bab Amro, they said. Activists said more than 200 people were killed on Friday night when tanks and artillery blasted the Khalidiya neighbourhood of Homs. It was the highest reported death toll in a single day since the uprising against Assad's rule, inspired by uprisings across the Arab world, erupted last March.Damascus denies firing on houses and says images of dead bodies on the Internet were staged. Reports from activists and authorities are hard to verify because Syria restricts access for independent media.The latest assault, which began shortly after 2 a.m. (midnight GMT) on Monday, appeared to be more widely targeted, with explosions in Khalidiya, Baba Amro, Bayada and Bab Dreib neighbourhoods, the activists said."They want to drive the Free Syrian Army out," said Bab Amro resident Hussein Nader by telephone, referring to the rebel force of army deserters and gunmen who have controlled parts of the city for months.In addition to those killed, 150 people had been wounded, he said. "Rockets are falling seconds apart on the same target."Al Arabiya reported that Syrian army deserters destroyed a military control post overnight, killing three soldiers and capturing 19.US Secretary of State Hillary Clinton said the United States would work with other nations to try to tighten "regional and national" sanctions against Assad's government "to dry up the sources of funding and the arms shipments that are keeping the regime's war machine going".All 13 other members of the Security Council voted to back the resolution, which would have "fully supported" the Arab League plan for Assad to cede powers to a deputy, a withdrawal of troops from towns and a start to a transition to democracy.Western anger at the veto was echoed by Syria's Middle East critics, including Arab powers Saudi Arabia and non-Arab Turkey who have turned against Assad in recent months.