AMMAN - A Syrian military assault on a rebel-held town has stalled because of snowstorms and armed resistance, an opposition leader said on Saturday, and the Arab League prepared to send more monitors to Syria. Tanks and troops attacked Zabadani, a town of 40,000 near the Lebanese border on Friday, but made little progress against army deserters and insurgents defending it, Kamal al-Labwani told Reuters, citing residents reached briefly by telephone. It was the biggest military assault against opponents of President Bashar al-Assad since Arab observers began work on December 26 to check Syria's compliance with an Arab peace plan. Security forces trying to crush anti-Assad protests killed 12 civilians, including a 6-year-old girl, on Friday, the British-based Syrian Observatory for Human Rights said. The state news agency SANA said armed "terrorists" had killed six soldiers or policemen. Violence has raged on despite the Arab observer mission, which has been castigated even by some of its own members for failing to halt the bloodshed and buying Assad time.