Activists say shooting has broken out during a visit to a Damascus suburb by an advance team of UN observers.
Videos posted online appear to show anti-government protesters ducking as snipers open fire in Arbeen while crowds are shown surrounding the UN cars.
The UN team is overseeing a shaky ceasefire brokered by UN-Arab League envoy Kofi Annan.
UN chief Ban Ki-moon is to ask the Security Council to expand the observer force from 30 to 250 members.
US Secretary of State Hillary Clinton said Syria was at a "crucial turning point".
The UN says about 9,000 people have died since pro-democracy protests began in March 2011.
Meanwhile, the wives of the German and UK ambassadors to the UN have posted a video on YouTube urging President Assad's wife, Asma, to help end the bloodshed in her country.