Twenty-five people were killed in army shelling in the central Syrian town of Halfayeh on Friday after a months-old local truce between the army and rebels fighters broke down, opposition activists said.
Halfayeh has been in rebel hands for more than five months and a truce was agreed there between the warring parties in an attempt to protect thousands of citizens, an activist from the region who called himself Safi al-Hamawi told Reuters.
But President Bashar al-Assad's forces issued an ultimatum to the town's elders saying the rebels must leave by Thursday evening and started shelling it heavily as the deadline passed, Hamawi said.
Amateur video footage posted on the Internet by opposition activists shows dozens of people wading through a river to escape the bombardment. A voice off camera says Halfayeh is under siege and this was the only route out to "escape a massacre" by government forces.
The Syrian Observatory for Human Rights, an anti-Assad group, said that it had confirmed five deaths but was working to corroborate other claims.
Assad, whose family has been in power for more than 40 years, is facing the greatest threat to his rule. What started as peaceful pro-democracy protests two years ago turned into a civil war after cracked down on opposition members.
Much of the country was lost to insurgents and Assad's grip on power was weakened, but government forces have launched a counter offensive in the past few weeks.
Halfayeh is the latest in a series of fronts that the army has opened up around the country, including in areas around the Lebanese border where pro-Assad Hezbollah Lebanese militants have been backing up the army.
UN High Commissioner for Human Rights Navi Pillay said in a statement on Friday that she was concerned by reports of a major military build-up by army and pro-Assad militia around Qusair, a town near the Lebanese border.
A resident who lives near Qusair said the army had dropped leaflets in the town on Friday urging civilians to leave before an assault.
Pillay said residents in Qusair clearly fear a possible repeat of last week's killings in the coastal village of Baida and the town of Banias in which more than 100 people, including babies, were killed in a government advance, according to opposition groups and video footage.
"I am appalled at the apparent killing of women, children and men ... which seem to indicate a campaign targeting specific communities perceived to be supportive of the opposition," Pillay said.
Activist reports on the killings could not be independently verified as the Syrian government restricts access for independent media.
International powers have been unable to halt the violence and some are actively arming the opposing sides.
Russia supplies Assad with weapons but agreed with the United States this week to bring the warring sides together for an international conference. Previous attempts have failed and neither Russia nor the US have hinted they will change policy.