GENEVA - The United Nations voiced concern on Friday at a military build-up near the rebel-held Syrian town of Yabroud, saying it feared a 'major assault' by government forces and stressed that Damascus had a legal duty to allow civilians to leave. 'We have received reports from within Syria that there have been numerous aerial attacks and shelling along with a military build-up around the town, suggesting a major assault by land may be imminent' on the town of 40,000-50,000 people, UN human rights spokesman Rupert Colville told a news briefing. Some 500-600 families have already fled and arrived in Arsal, Lebanon, citing 'their fear of this attack' and the UN refugee agency is 'bracing for a big influx' across the Lebanese border, UNHCR spokeswoman Melissa Fleming told reporters.