DAMASCUS - Syrian rebels braced on Saturday for a new assault on their beleaguered stronghold of Qusayr by the army and its Hezbollah allies, who were bolstering positions north of the town, a watchdog said.

UN Secretary General Ban Ki-moon demanded both sides allow civilians to flee. In New York, he urged “all sides to do their utmost to avoid civilian casualties,” spokesman Martin Nesirky said in a statement. The UN’s rights chief Navi Pillay and emergency relief coordinator Valerie Amos also said they were “extremely alarmed” by reports that civilian neighbourhoods were under fire.

The opposition Syrian National Coalition saluted rebels in Qusayr. “The heroes of the Free Syrian Army prove every day that they are worthy of the responsibility that the people have entrusted them with,” it said.

The powerful Lebanese group, Hezbollah, a staunch ally of President Bashar al-Assad’s regime, has dispatched thousands of fighters to help put down the uprising that began more than two years ago with peaceful protests. Some members of Lebanon’s Sunni Muslim community have also crossed the border to fight alongside the Sunni-led rebels, encouraged by local clerics.

Late on Friday, influential Muslim cleric Yusuf al-Qaradawi urged Sunnis to follow suit and join the uprising. “Every Muslim trained to fight and capable of doing that (must) make himself available” to support the Syrian rebels, he said at a rally.

“Iran is pushing forward arms and men (to back the Syrian regime), so why do we stand idle?” added Qaradawi, a controversial figure who has millions of supporters, mostly from the Muslim Brotherhood.

Despite an official policy of neutrality on the conflict, Lebanon has found itself increasingly embroiled in its neighbour’s civil war. On Saturday morning, at least six rockets fired from Syria struck the Bekaa region, causing no injuries.