An Associated Press reporter who was traveling in the U.N. convoy said the explosion blew out the military truck's windows and caused a plume of thick, black smoke. The U.N. convoy was not hit.
"We were driving behind the U.N. convoy as protection when a roadside bomb exploded, wounding a 1st Lieutenant and five troops," a soldier who asked to be identified only by his first name, Yahya, told The Associated Press at the scene.
At least three bloodied soldiers were rushed away.
The blast went off after the head of the U.N. observer mission, Maj. Gen. Robert Mood, headed into this southern Syrian city with a team of observers and a convoy of journalists. The explosion was more than 100 meters (330 feet) behind the convoy.
It's not clear who was behind the blast.
But Syria's rebel leader, Col. Riad al-Assad, threatened to resume attacks because the government has not honored a cease-fire, the London-based Asharq al-Awsat newspaper reported Wednesday. Al-Assad told the paper that "our people are demanding that we defend them."
The comments were published in Wednesday's edition of the paper and could deal yet another blow to a peace plan brokered by special envoy Kofi Annan.
On Tuesday, Annan gave a bleak assessment of the crisis in Syria, saying violence remains at "unacceptable levels" and warning that his peace plan is the country's last chance to avert a disastrous civil war.
Annan insisted there is still hope and said the presence of U.N. observers has had a calming effect on the crisis, which has killed at least 9,000 people since March 2011.
"There is a profound concern that the country could otherwise descend into full civil war and the implications of that are frightening," Annan told reporters in Geneva after briefing a closed-door session of the U.N. Security Council in New York by videoconference. The observer mission, he said, "is the only remaining chance to stabilize the country."
Syria has become one of the bloodiest conflicts of the Arab Spring, and world powers have been unable to stop the violence. Syrian President Bashar Assad still has a firm grip on power, and his regime portrays his opponents as terrorists out to weaken the country.
Although the death toll mounts daily, the U.N. has ruled out any military intervention of the type that helped bring down Libya's Moammar Gadhafi, in part out of fears that it could make the conflict worse. Syria is an important geopolitical linchpin with a web of allegiances to powerful forces, including Lebanon's Hezbollah and close ally Iran.