Two powerful explosions have rocked Damascus as U.N. monitors try to enforce a shaky cease-fire between the Syrian government and rebels.

Witnesses say the blasts, which occurred early Thursday, sent plums of smoke billowing into the air, but the cause of the explosions was not immediately clear.

In another development, activists reported intense government shelling in the flashpoint Homs region on Thursday.

On Wednesday, a roadside bomb hit a convoy that included U.N. monitors who were heading to the southern city of Dara'a. The monitors were not harmed but the Syrian government said 10 soldiers acting as escorts were wounded.

Observer mission leader Norwegian Major General Robert Mood was part of the convoy and condemned the attack. He is quoted by his spokesman as saying it was “an example of what the Syrian people were suffering on a daily basis” and that “all forms of violence must stop.”

A statement from U.N. chief Ban Ki-moon's office said the bombing, and other acts of violence in Syria, “call into question the commitment of the parties to the cessation of violence.”

U.N. observers have been fanning out across Syria to monitor compliance with the cease-fire, which is part of a peace plan negotiated by international envoy Kofi Annan.

The government has blamed armed terrorist groups for much of the unrest.

The United Nations says more than 9,000 people have been killed in violence related to the anti-government uprising which erupted more than a year ago.

Meanwhile, vote-counting continues from Monday's parliamentary elections in Syria. Opposition groups have dismissed the elections as a sham.