ALEPPO (Agencies): A Syrian army helicopter fired machinegun rounds and troops shelled rebel positions in Aleppo on Saturday, a Reuters witness said, as they tried to break through the insurgents' frontline in a battleground district in the country's largest city.Earlier in the day, Syrian forces clashed with rebels around Aleppo's television and radio station, activists said, and a local rebel commander said his fighters were preparing for a "strong offensive" by government forces on the city.Syrian troops backed by armor stormed the last opposition bastion in Damascus on Friday in a drive to crush a rebel offensive that coincided with a bomb attack that killed four of President Bashar al-Assad's senior security officials.The onslaught continued on Saturday as jets bombarded the capital in a bid to snuff out resistance, a resident said.Syria's civil war has intensified in the last few weeks, with fighting engulfing Damascus and Aleppo for the first time in the 17-month-old uprising against Assad family rule.The two cities are crucial prizes for both sides in an increasingly brutal struggle that has eluded all attempts at a diplomatic solution and risks igniting a wider conflagration.U.N. member states on Friday voted overwhelmingly to condemn the Syrian government at a special session of the 193-nation General Assembly that Western diplomats said highlighted the isolation of Assad supporters Russia and China.Western and Arab powers want Assad to step aside but Russia and China have used their Security Council vetoes to block attempts to force him out. They say outside interference is prolonging the bloodshed.In Aleppo's battleground Salaheddine district, rebels from the Free Syrian Army hid in alleyways, dodging the Syrian army's bullets and tank rounds that struck a building in the district on Saturday."There is one helicopter and we're hearing two explosions every minute," a Reuters reporter said.