At least 20 people have been killed by Syrian security forces in the north-western town of Khan Sheikhoun during a visit by UN observers, activists say.
Unconfirmed reports say the deaths occurred when security forces opened fire on a funeral procession in the town in Idlib province.
Three of the monitors' cars were damaged in a blast in the town, but the observers were not hurt, the UN said.
This comes despite the UN-backed ceasefire in place since last month.
The Syrian government has so far not publicly commented on the incidents.
In a separate development, the head of Syria's election committee announced that turnout in last week's parliamentary elections was more than 50%.
The committee did not say who had won, but it is clear that the ruling Baath party has secured a substantial majority of the seats, the BBC's Jonathan Head in neighbouring Turkey reports.
This was the first election in which the Baath party was not, in theory, guaranteed a majority, our correspondent says.
Opposition parties - which boycotted the election - have dismissed the vote as a sham.
As the parliament's powers are poorly defined, he adds, there was no chance that the poll would dilute President Bashar al-Assad's power, our correspondent says.
"The Syrian regime committed a massacre [on] Tuesday during a visit by UN monitors to Khan Sheikhoun," the UK-based Syrian Observatory for Human Rights said on Tuesday, according to the AFP news agency.
The group also called for an international investigation into the attack.
Separately, a spokesman for the UN-Arab Union envoy to Syria, Kofi Annan, said the UN vehicles were damaged in Khan Sheikhoun.
Spokesman Ahmad Fawzi said the UN convoy was struck by "an explosion from an improvised explosive device".
He added that the UN mission in Syria sent a patrol team to the area to evacuate the monitors.
The team of seven UN observers was in the town to observe a demonstration by opposition supporters at the funeral.
UN monitors were also hit by gunfire during an intense battle in the town of al Rastan on Monday, and the convoy of another UN team was struck by an explosion in Deraa last week.
The BBC's Jonathan Head, in neighbouring Turkey, says that despite the announced ceasefire the levels of violence are steadily creeping up and may reach the level when the UN monitors can no longer operate.
The UN said over the weekend that it had 189 observers in Syria, some two-thirds of the total intended for deployment as part of a six-point peace plan mediated by Mr Annan.
Earlier this week, at least 30 people - including 23 soldiers - reportedly died in clashes in central Syria, in what would be one of the deadliest suffered by security forces in the 14-month-long uprising against President Assad.
The government in Damascus says it is fighting organised gangs.
The UN estimates at least 9,000 people have died since pro-democracy protests began in March 2011.