An explosion has hit a Syrian military truck escorting a convoy of UN observers near the city of Deraa, just seconds after UN staff had 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. The windows of the truck were shattered.
The UN Secretary General, Ban Ki-moon, said such incidents could threaten the future of the UN mission in Syria.
The observers are in the country as part of the joint UN-Arab League peace plan and began deploying last month.
There are now about 70 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. However, the opposition Syrian National Council is blaming the government, saying the explosion is part of a campaign to drive the UN monitors out of the country.
Maj Gen Mood condemned the blast, saying the observers would remain focused on their mission.
"This is a graphic example of violence that Syrians do not need. It is essential to halt the violence in all its forms," his spokesman told a news conference in Deraa, according to the AFP news agency.
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, AFP reported.
Violence was reported in several parts of the country on Wednesday, including the northern province of Idlib and the city of Hama.
Two members of the security forces were killed by gunmen in the eastern town of Deir al-Zour, according to the UK-based Syrian Observatory for Human Rights.
Also on Wednesday, Syrian troops fired across the border into Lebanon, killing an elderly woman and wounding her daughter, Lebanese officials said.
The UN estimates about 26,000 Syrians have fled across the border to Lebanon, most of them in the north.
On Tuesday, UN-Arab League envoy Kofi Annan told the Security Council that his peace plan could be the "last chance to avoid civil war" in Syria.
He told a closed session that the plan was "not an open-ended commitment" and highlighted continuing violations.
Mr Annan said he was particularly concerned that torture, mass arrests and other human rights violations were "intensifying".
He also told the council that President Assad bore "primary responsibility" for ending the military campaign.
The UN says at least 9,000 people have died since pro-democracy protests began in March 2011. In February, Syria's government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.
Syria heavily restricts access to foreign journalists and the reports cannot be independently verified.