At least 20 people have been killed by Syrian security forces in the town of Khan Sheikhoun during a visit of UN observers, activists say. Unconfirmed reports say the deaths occurred when security forces opened fire on a funeral procession in the north-western town, Idlib province. Three of the monitors' cars were damaged in a blast in the town, but the observers were not hurt, the UN said. It comes despite a UN-backed ceasefire in place for just over a month. "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, 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. He did not give any further details. The Syrian government has so far not publicly commented on the incidents.