A bomb struck a Syrian military convoy escorting the head of a U.N. observer mission Wednesday, injuring several Syrian soldiers and illustrating the vulnerability of the unarmed and unprotected mission as it struggles to monitor a badly fraying cease-fire.

None of the U.N. monitors accompanying their chief, Maj. Gen. Robert Mood of Norway, was harmed, and the convoy continued on its way toward the southern town of Daraa, according to a statement from the U.N. Supervision Mission in Syria.

However, several Syrian soldiers in the rear escort vehicle of the convoy suffered injuries and were hospitalized, the statement said. Syrian media said eight soldiers were injured.

“This was a graphic example of what the Syrian people are suffering on a daily basis and underlines the imperative for all forms of violence to stop,” Mood was quoted as saying.

A spokesman for the opposition Free Syrian Army denied responsibility for the attack, saying rebel fighters would “never” target members of the U.N. mission.

But Col. Malik Kurdi, speaking from a refugee camp in southern Turkey, warned that the rebels are not prepared to continue observing a cease-fire that he said the government is refusing to respect. Since the truce went into effect April 12, rebels have conducted only “defensive” operations, but that will soon change, he said.

“From now on, we will carry out preventive operations because the regime did not stop its attacks and the observers’ mission has been a cover for more killing,” he said. “We have to do what is necessary to defend our people.”

It is unclear how much authority the Free Syrian Army’s leadership in Turkey has over the ad hoc assortment of rebel units and militias that have sprung up spontaneously throughout the country in response to the government’s harsh crackdown on a 14-month-old uprising.

In a letter delivered to the United Nations on Tuesday, the Syrian government listed what it said were more than 1,200 truce violations by the rebels, who it said had never halted their attacks.

Activist groups say the government has sustained attacks against opposition strongholds with gunfire and artillery, in violation of the cease-fire. The Local Coordination Committees, an opposition group, said 15 people were killed in government attacks Wednesday, bringing the death toll in the month since the shaky truce took hold to 1,025.

The claims and counterclaims are impossible to verify because the government continues to deny visas to many news organizations and imposes restrictions on those operating in Syria.

Overall levels of violence have fallen since the cease-fire took effect, though it is becoming increasingly clear that the U.N. plan is failing to end the bloodshed or to create the conditions for its ultimate goal: negotiations on a transition to democracy.

But no one has yet proposed an alternative to the six-point plan, brokered by special envoy Kofi Annan, which has sharply divided the international community. Russia continues to wholeheartedly back the plan as the best chance for ending the conflict, while the United States has made it clear that it believes the cease-fire has failed.

Annan, a former U.N. secretary general, warned Tuesday that violence remains at “unacceptable levels,” but he pleaded for the plan to be given more time to avert a potentially devastating civil war.

“If it fails . . . and it were to lead into a civil war, it will not affect only Syria,” he told reporters in Geneva. “It will have an impact on the whole region.”

The U.N. mission is still not up to strength, he added, and “we will see much greater impact” when it is.

The observer mission currently consists of 70 military monitors and 43 civilians, based in five Syrian locations, the U.N. statement said. The mission is not expected to reach its mandated strength of 300 observers until the end of the month.