An explosion has hit a Syrian military truck escorting a convoy of UN observers near the city of Deraa, just seconds after the observers passed by.
The head of the UN team, Maj Gen Robert Mood, was in the convoy, but neither he nor any of the other monitors was hurt.
Eyewitnesses said at least three Syrian soldiers were wounded. One report said the truck windows were shattered.
The observers are in Syria as part of the joint UN-Arab League peace plan and began deploying last month.
There are now about 30 monitors in Syria but their presence has had no effect in quelling the violence.
Deraa was where the uprising against the government of President Bashar al-Assad began in March 2011.
'Dance of death'
An Associated Press reporter travelling in the UN convoy said the explosion blew out the military truck's windows and caused a plume of thick, black smoke.
It is not clear who was behind the blast.
Meanwhile, the BBC's Lyse Doucet, who has been to the Syrian city of Homs, says there is constant shooting there, despite a ceasefire between government and opposition forces.
Our correspondent, who spent the night in Homs, also heard heavier weapons being fired.
She saw UN observers patrolling the city but said entire neighbourhoods were deserted.
In other developments, UN Middle East envoy Terje Roed-Larsen told the Security Council that arms were being smuggled in both directions between Lebanon and Syria.
"What we see across the region is a dance of death at the brink of the abyss of war," he told reporters later, the AFP news agency reported.