BEIRUT — Syrian President Bashar Al-Assad urged his military Wednesday to boost its fight against rebels, but his written call to arms only deepened a mystery over his whereabouts two weeks after a bomb penetrated his inner circle. Assad has not spoken publicly since the July 18 bombing killed four of his top security officials — including his brother-in-law — during a rebel assault on the capital, Damascus. The president's low profile has raised questions about whether he fears for his personal safety as the civil war escalates dramatically. The US called the Syrian president a coward for marshaling his forces from the pages of the army's official magazine. “We think it's cowardly, quite frankly, to have a man hiding out of sight, exhorting his armed forces to continue to slaughter the civilians of his own country,” said US State Department spokesman Patrick Ventrell. Sausan Ghosheh, the spokeswoman for the UN mission in Syria, said international observers witnessed warplanes firing in Aleppo, Syria's largest city, where intense fighting has been raging for 12 days. Speaking to reporters in Damascus, Ghosheh said the situation in Aleppo was dire. “Yesterday, for the first time, our observers saw firing from a fighter aircraft. We also now have confirmation that the opposition is in a position of having heavy weapons, including tanks,” she said, adding that for civilians, there “is a shortage of food, fuel, water and gas.” Explosions could be heard and helicopter gunships cruised the skies as government forces tried to push the rebels out of the historic city and preserve one of Assad's main centers of power. Earlier, at least 10 volleys of shells lit up the darkened sky and carloads of rebel fighters shouting “God is Greatest” sped off toward the fighting. Aleppo battle has become a crucial test for both sides. Neither Assad's forces nor the rebels can afford to lose if they hope to prevail in the wider struggle for Syria. Meanwhile, video footage posted on the Internet appeared to show that rebel fighters were carrying out summary executions in Aleppo in much the same way as government forces have been accused of. One video showed four men identified as members of the pro-Assad Shabbiha militia being led down a flight of stair, lined up against a wall and shot in a hail of rifle fire. In another video, a cameraman filmed the bodies of about 15 men lying dead at a police station. One rebel fired at the corpse of the station commander, blowing his head off. According to an NBC News report, the rebels have acquired nearly two dozen surface-to-air missiles, which were delivered to them via neighboring Turkey. The missiles could tilt the battlefield balance if the rebels were able to shoot down government helicopter and war planes. — Agencies