DARAA, Syria — A roadside bomb hit a Syrian military truck Wednesday just seconds after the head of the U.N. observer team drove by in a convoy, demonstrating the fragility of the international plan to end the country's bloodshed.
In Washington, meanwhile, President Barack Obama took steps to extend sanctions against the government of President Bashar Assad, saying Syria poses an "unusual and extraordinary threat" to U.S. national security and diplomatic goals.
The attack, which the regime said wounded 10 Syrian soldiers, emphasized the limits of the international community's plan to use unarmed observers to promote a cease-fire between government troops and rebels trying to topple Assad.
The team of 70 U.N. military observers now in Syria should grow to more than 100 in the coming days. It is unclear when the full team of 300 will arrive. They are to oversee a U.N.-brokered cease-fire that was intended to allow for talks on a political solution to the conflict but began unraveling shortly after it was due to take effect on April 12.
U.N. Secretary-General Ban Ki-Moon condemned the attack, saying the continuing violence undermines the plan, which is "possibly the only remaining chance to stabilize the country and avert a civil war."
The peace plan has been troubled from the start, with government troops shelling opposition areas and rebels attacking military convoys and checkpoints despite the cease-fire.
Many civilians have grown critical of the plan, saying it does not protect them from regime forces.
U.S. State Department spokeswoman Victoria Nuland said the roadside blast that hit troops escorting the U.N. monitors was "further evidence that the cease-fire is not holding."
Although the daily death toll has dropped in recent days, international envoy Kofi Annan said Tuesday that the level of violence is unacceptable and that the plan's failure could lead to civil war.
Wednesday's blast, witnessed by an Associated Press reporter who was traveling with the United Nations, provided a close-up example of the attacks on security forces that have become almost daily events.
The bomb went off as Maj. Gen. Robert Mood, the Norwegian head of the U.N. observer team, rode in a clearly marked armored white Land Cruiser from the capital, Damascus, to the southern city of Daraa, where Syria's uprising began.
The explosion, which occurred at about 11:20 a.m. after the convoy had passed a military checkpoint, shattered the windows of the Syrian military truck and sent up a cloud of smoke and red sand. The truck sped into the city, where several bloodied soldiers were rushed to a hospital.
Speaking to reporters later, Mood said it was unclear whom the bomb was targeting.
"For me the important thing is really not speculating about who was the target, what was the target, but it is to make the point that this is what the Syrian people (are) seeing every day and it needs to stop," he said. "Whoever is doing it and whoever is supporting it."
No one claimed responsibility for the bombing. The regime blames such attacks on terrorists it says are stoking the anti-Assad uprising.
An exiled rebel leader, Col. Riad al-Asaad, warned that armed groups in the country would resume attacks because the government had flouted the cease-fire, the London-based Asharq al-Awsat newspaper reported Wednesday. Al-Asaad told the paper that "our people are demanding that we defend them.
The streets of Daraa were crowded on Wednesday and stores were open, but the city felt tense amid heavy security, with soldiers deployed everywhere and at least three checkpoints leading to the city center. Long lines of drivers waited to have their IDs checked or cars searched.
Dozens of soldiers stood outside the hotel where some observers have been stationed, keeping a close eye on nearby buildings.
Activists said regime forces fired a mortar shell at the home of an activist in Daraa soon after he met with U.N. observers there on Wednesday, injuring 10 people.
"Each time they come, there is an attack," activist Karam al-Hariri said by phone.
Another activist, Mohammed Abu Hawran, said the observers returned after the attack to take pictures and check on the wounded.
An amateur video posted online showed the tail of an exploded mortar shell on a street with blood spattered on a nearby wall and sidewalk. Activist claims and videos could not be independently verified.
Syria's conflict started in March 2011 with mass protests calling for political reform. The government swiftly cracked down, dispatching tanks, troops, snipers and pro-government thugs to quash dissent, and many members of the opposition took up arms to defend themselves and attack government troops. Many soldiers also switched sides.
The U.N said weeks ago that more than 9,000 people had been killed. Hundreds more have died since.
International diplomacy has failed to stop the violence, and the U.N. has ruled out military intervention of the type that helped bring down Libya's Moammar Gadhafi, in part out of fear that it could exacerbate the violence.
On Wednesday, Obama notified Congress that he is extending a "national emergency" that allows the president to impose a variety of punishments and controls against Syria. The renewal came two days before the state of emergency was set to expire.
An amateur video posted online Wednesday showed a U.N. observer telling a resident in the town of Talbiseh that the team had come to investigate a recent attack and file a report.
The resident said that was not enough, that the conflict has been going on for over a year and there were still army checkpoints around the city. Another video Wednesday from the same city showed the observers walking up to a government tank.
"Why hasn't the army evacuated?" the unidentified resident asked. "The first clause of the Annan plan is that the army withdraw from cities. So where is the army withdrawal?"
Hubbard reported from Beirut.