An explosion occurred outside the premises of a state-controlled labor union behind a hotel in the Syrian capital Damascus on Sunday, causing casualties, state television said.

The blast, which the television described as "terrorist" - the government's word for rebels fighting to topple President Bashar al-Assad - took place in the union's parking lot. It gave no more immediate details.

"The terrorist explosion caused several injuries. One of (the injured) is in a critical condition," the television said.

Opposition activists told Reuters the explosion was near heavily-guarded army installations in the heart of the capital.

Rebels have carried out a series of bombings targeting government and military buildings in Damascus this year, bringing the civil war to the heart of Assad's power.

Islamist militant groups Ahfad al-Rasoul and Ansar al-Islam said last month they had planted bombs in a major state security compound in Damascus. They also claimed responsibility for suicide car bombs earlier in October that hit a security compound on the edge of Damascus.