At least 21 civilians were killed when Syrian security forces opened fire on a funeral procession in the central town of Khan Sheikhoun, in Idlib province, during a visit by the 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.
A spokesman for 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.
One monitor 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 special envoy Kofi Annan, had no other details.
An Idlib-based activist, Fadi al-Yassin, said he witnessed the attack on the funeral and saw that the observers were there.
'On the brink'
Speaking from New York, Kieran Dwyer, spokesman for the United Nations, said that the UN mission headed by joint UN-Arab League envoy, Kofi Annan, to end the violence in Syria is not working.
Citing concern for the communities on the ground, Dwyer, said "Syria is on the brink of far-reaching civil war if all parties don't step back from the violence", but that, per their mandate, the observers are not able to prevent violence through any show of force.
Responding to criticism that violence has not ceased in the month that observers have been in the nation, Dwyer told Al Jazeera "the violence was taking place before the UN went in there ... to say that somehow we are covering that up is a silly analysis", and that any future UN actions in Syria are up to the discretion of the United Nations Security Council (UNSC).
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