Syrian army air strikes in the central city of Homs on Sunday killed three civilians, as regime forces pressed forward with a new bid to to retake several rebel-controlled districts, activists said. \"Syrian warplanes carried out air strikes on the Old City of Homs a little while ago, destroying a house and causing three deaths,\" the Syrian Observatory for Human Rights said. \"Regime forces also carried out heavy shelling of the districts of Khaldiyeh and the Old City, and the sound of explosions could be heard,\" the group added. Observatory director Rami Abdel Rahman said at least three civilians, a woman and two children, were killed in the air strikes in the Old City, and that dozens of people had been injured since the army assault began on Saturday. \"The army is continuing its attempt to enter Khaldiyeh, but it hasn\'t succeeded so far,\" he said. On Saturday, the group said regime forces were engaged in \"unprecedented\" bombardment of several parts of Homs, including Khaldiyeh, Bab Hud, Hamidiyeh and Bustan al-Diwan. A security source in Damascus confirmed fierce fighting was underway in Homs. \"Military operations never stopped in Homs, but their pace increases according to priorities,\" he told AFP on Saturday. \"What\'s important is to cleanse those neighbourhoods of Homs that are in the hands of armed terrorists, particularly Khaldiyeh, Hamidiyeh and the Old City,\" he added. Syria\'s Al Watan daily, which is close to the government, said on Sunday that the army had \"made qualitative new advances in the city of Homs amid fierce clashes with armed militias in Khalidiyeh and Bab Hud.\" The operation prompted Syria\'s key opposition National Coalition to issue a statement late Saturday urging \"battalions of the Free Syrian Army to come to the aid of Homs with all means possible\". The group also called on international backers of the uprising to establish a no-fly zone and carry out air strikes against regime military bases. Homs, the third largest-city in the country, was one of the first to join the revolt against President Bashar al-Assad\'s regime more than two years ago. Khaldiyeh and the Old City have been under siege by the army for around a year.