Syrian troops backed by fighters from Lebanese militant group Hezbollah launched an assault on the rebel-held central town of Qusayr on Sunday, after months of fierce fighting in the area, a watchdog said. The Syrian opposition condemned "attempts to invade" the town, which it said could render US-Russian attempts to organise a peace conference "meaningless." "The assault on Qusayr has started. There is fierce fighting between rebels and the army around the entrances to the town," Syrian Observatory for Human Rights director Rami Abdel Rahman said.