Activists say 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.
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.
This comes despite the UN-backed ceasefire in place since last month.
The Syrian government has so far not publicly commented on the incidents.