The United Nations says members of its observer team trying to reach the town of Al-Haffa in northern Syria on June 12 were attacked by angry crowds of people who threw stones and metal bars at them.
Shots were later fired at the UN vehicles, but none of the observers were injured.
The UN said the crowds appeared to be residents of the area but it was not clear who fired the shots.
Al-Haffa is close to Kardaha, the hometown of President Bashar al-Assad's family.
Government forces have been bombarding positions held by rebels against Assad's regime in Haffa for over a week.
UN peacekeeping chief Herve Ladsous said on June 12 that Syria is now in full-scale civil war as government forces are trying to recapture areas they lost to the rebels.
U.S. Secretary of State Hillary Clinton said on June 12 the United States is worried Russia is sending Syria attack helicopters that would dramatically escalate the 15-month old conflict in Syria.
"We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria which will escalate the conflict quite dramatically," Clinton said.
Clinton, in an appearance at the Brookings Institution, said Russian claims that arms shipments to Syria were unrelated to the Syrian government's crackdown on rebels was "patently untrue."
"We have confronted the Russians about stopping their continued arms shipments to Syria. They have from time to time said that we shouldn't worry, everything they're shipping is unrelated to their actions internally. That's patently untrue," Clinton said.
Clinton also said that without progress on implementing a UN-backed peace plan, it would be very difficult to extend the UN observer mission in Syria.
The UN mission has a 90-day mandate that expires on July 20.
UN special envoy for Syria Kofi Annan has asked governments with influence to "twist arms" to implement the six-point peace plan, according to his spokesman.
The spokesman, Ahmad Fawzi, did not mention the countries that, according to Annan, might be able to pressure Assad into halting his onslaught against the opposition.
Based on reporting by AP and Reuters