At least 21 civilians have been killed after Syrian security forces opened fire on a funeral procession in Khan Sheikhoun, a town in Idlib province, during a visit by UN peace monitors, activists say.

A spokesman of the rebel military council gave a higher death toll, saying at least 50 people were killed in Tuesday's attack during which cars belonging to the UN team were hit.

The activists said the observers were not among the wounded, but their vehicles were damaged.

An Idlib-based activist, Fadi al-Yassin, said he witnessed the attack on the funeral and saw that the UN observers were there.

A spokesman for the UN mission in Syria said the "convoy of four vehicles was struck by an improvised explosive device".

"Three UN vehicles were damaged. No UN personnel were injured. The mission has sent a patrol team to the area to extract the UN military observers," the spokesman said.

'Safe with Free Army'

One observer and a member of the Free Syrian Army (FSA) said the seven-strong team of observers was with FSA fighters.

"We are safe with the Free Army and we are waiting for a [UN] group to pick us up," the monitor told Reuters news agency.

Ahmad Fawzi, a spokesman for Kofi Annan, the UN special envoy, had no other details.