The monitors said by telephone that they were safe - they were under the "protection" of the Free Syrian Army who attacked the convoy and killed 21. None of the monitors were injured.
Each side blamed the other for the attack in Khan Sheikhoun in northern Idlib province.
Some rebel and opposition sources put the death toll from the attack as high as 66.
Pro-government Addounia TV said gunmen had opened fire on the monitors, but did not mention casualties.
The monitor who spoke to Reuters said gunfire had erupted as a seven-man U.N. team toured Khan Sheikhoun, then a blast damaged one of the group's vehicles.
Ahmad Fawzi, international mediator Kofi Annan's spokesman, said the convoy was hit by an improvised explosive device.
"Three U.N. vehicles were damaged but no U.N. personnel were hurt in this explosion. The mission has sent a patrol team to the area to help to extract those U.N. military observers," he said in a statement.
Internet footage appeared to show a white vehicle like those used by monitors with a damaged front. In Damascus Major General Robert Mood, the head of the U.N. monitoring mission, told reporters the team was safe, without elaborating.
A British-based opposition group, the Syrian Observatory for Human Rights, said government troops had opened fire on a funeral procession in the town, about 220 km (140 miles) north of Damascus.
The group said a total of 46 people had been killed by government forces across the country. There was no independent confirmation, and Syria has limited journalists' access during the uprising.
Neither side really wants the UN monitors at this point. They're just getting in the way of both sides trying to kill each other.
The US has said that they will not reauthorize the monitor's 90 day mission that will end in July. It may be unnecessary because if the Syrian government can't keep the monitors safe, it is likely that the Security Council will pull the plug on the operation and bring the monitors home.