By Carlo Muñoz - 05/15/12 07:27 PM EDT
UN officials confirmed to The Associated Press that a four-vehicle convoy was hit by an improvised explosive device while traveling through the town of Khan-Sheikhoun in northern Syria.
Tuesday's strike was the second time international observers were involved in an IED attack.
A Syrian military truck attached to another UN convoy was hit by a roadside bomb in the southern part of the country last week. The UN observation team was unharmed in that attack, according to the AP.
Annan announced in April that 300 observers would be brought into Syria to oversee a cease-fire between troops loyal to Syrian President Bashar al-Assad and anti-government troops.
The cease-fire was part of a six-point peace plan brokered by Annan and agreed to by Assad in March. But Tuesday's violence could be seen as just the latest indication that Annan's plan for Syria is failing.
Earlier this month, White House press secretary Jay Carney said it might be time for international advocates of the Annan peace plan to "admit defeat" and begin considering other options.
"It is clear ... that the plan has not been succeeding thus far,” Carney said during a May 3 briefing at the White House. The Pentagon has already begun exploring post-Assad scenarios with its NATO partners.
Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey met with alliance leaders in late April to discuss the possible political and military fallout in Syria if Western powers decide to remove Assad by military force.
American and European military leaders essentially "want to know what [comes] next, before taking that step [to] military action," Dempsey said during his May 2 speech at the Carnegie Endowment for International Peace.
Turkey's recent threat to invoke Article 5 of the NATO charter only ratcheted up that growing pressure for military force in Syria.
Article 5 claims that an attack against one NATO member can be considered an attack on the entire alliance.
Invoking Article 5 could open the door for a NATO-led attack on Assad's forces, similar to the campaign that removed former Libyan dictator Moammar Gadhafi.