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.
There was no claim of responsibility for Thursday's blasts. But an al Qaeda-inspired group has claimed responsibility for several past explosions, raising fears that terrorist groups are entering the fray and exploiting the chaos.
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 bloodshed, 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.
Special envoy Kofi Annan brokered a peace plan last month, but the initiative has been troubled from the start, with government troops shelling opposition areas and rebels attacking military convoys and checkpoints after the cease-fire was supposed to begin on April 12. Many civilians have grown critical of the plan, saying it does not protect them from regime forces.
Although the daily death toll has dropped in recent days, Annan said Tuesday that the level of violence is unacceptable and that the plan's failure could lead to civil war.
A team of 70 U.N. military observers now in Syria should grow to more than 100 in the coming days. A full team of 300 is expected by the end of the month to oversee a cease-fire intended to allow for talks on a political solution to the conflict.
On Wednesday, a roadside bomb hit a Syrian military truck in a southern province just seconds after the head of the U.N. observer team was driving 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 Assad's government, saying Syria poses an "unusual and extraordinary threat" to U.S. national security and diplomatic goals.