A roadside bomb struck a Syrian military truck near Deraa, wounding six soldiers just seconds after a convoy carrying the head of the UN observer mission passed by.
An Associated Press news agency reporter who was travelling in the UN convoy said the explosion blew out the military vehicle's windows and sent out a plume of black smoke. Vehicles in the UN convoy were not hit.
Major-General Robert Mood, the head of the UN mission, was in the convoy but escaped unharmed along with 11 other observers, said an AFP photographer travelling with them.
The Norwegian officer said the attack was "a graphic example of violence that the Syrian people do not need".
"We were driving behind the UN convoy as protection when a roadside bomb exploded, wounding a first lieutenant and five troops," a soldier who asked to be identified only by his first name, Yahya, told AP at the scene.
At least three bloodied soldiers were rushed away.
The blast went off after the head of the UN observer mission headed into the southern Syrian city with a team of observers and a convoy of journalists.
The explosion was more than 100 metres behind the convoy. It was not clear who was behind the blast.
Al Jazeera’s Rula Amin, reporting from Beirut in neighbouring Lebanon, said: "We heard some news coming from Free Syrian Army claiming responsibility for the attack, saying they did attack a military vehicle in Deraa, but it was a vehicle with no UN observers.
"How will this affect the UN mission and their free access is not clear."
Bernard Valero, French foreign ministry spokesman, strongly condemned the attack. "We hold the Damascus regime responsible for the observers' security," he said in Paris.
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Meanwhile, troops pounded a rebel hideout near Damascus killing at least three people on Wednesday, among them two soldiers and one civilian, the UK-based Syrian Observatory for Human Rights (SOHR) said.
Residents of Douma, about 13km from Damascus, reported heavy shelling since dawn and bursts of gunfire in the town, the SOHR said.
Rebels fighting to overthrow the government of President Bashar al-Assad have apparently sought refuge in Douma.
In Damascus itself, clashes erupted briefly between a security patrol and rebels in the neighbourhood of Maisat, but there were no casualties, the SOHR said.
In the eastern province of Deir Ezzor, security forces carried out raids and arrests in the villages of Al-Safira and Al-Hisan.
Two members of Syrian security forces were killed after midnight in the Jura district of Deir Ezzor, the scene of heavy shooting and explosions, the watchdog reported.
On Tuesday evening, a man was killed in the town of Soura in Deir Ezzor after unidentified gunmen attacked his car, leaving his daughter wounded.
Reports of violence
One civilian was killed and three wounded in heavy machinegun fire by regime forces in Tell Ain al-Hamra, near the town of Jisr al-Shughur in the northwest province of Idlib, on Wednesday.
"Jisr al-Shughur is near the Turkish border and has had a significant presence of rebels since the beginning of the revolt," the Observatory's Rami Abdel Rahman told AFP.
In-depth coverage of escalating violence across Syria
Rural villages east of Jisr al-Shughur and the town itself came under mortar attack by security forces, he added.
In the same province, in the village of Ahsem, an army checkpoint was targeted by an explosion followed by gunfire, the Observatory reported.
Shots were also heard early on Wednesday in several districts of the central Syrian city of Homs, a stronghold of opposition groups.
Kofi Annan, the international peace envoy, said on Tuesday that his peace plan - it went into effect April 12 but has since been regularly violated by both sides to the conflict - was "probably the last chance to avoid civil war" in Syria.
Nearly 12,000 people, mostly civilians, have been killed in Syria since an anti-regime uprising erupted in mid-March last year, according to the Observatory.