Mortar shells have slammed into a cafeteria at Damascus University, killing at least 12 people and wounding 20, according to state media and an official. It was the deadliest in a string of such attacks on President Bashar Assad's seat of power.

Rebels began firing shells at the capital earlier this year, and the strikes have become increasingly common as rebels clash with government troops on the city's east and south sides.

State-run TV said 12 people were killed when mortar shells struck the cafeteria of the university's architecture department in the central Baramkeh district. A Syrian official 20 people were wounded.

There was no immediate claim of responsibility for the attack, which came two days after rebels barraged Damascus with mortar shells that killed at least three people and wounded dozens.

The shelling rarely causes many casualties, but it has shattered the aura of normality the regime has tried to cultivate in Damascus.

The government blamed "terrorists," the term it uses for rebels fighting to oust Mr Assad, and called the attack as a "barbaric massacre."

Government-run Al-Ikhbariya TV station showed footage of plastic tables and chairs turned upside down, shattered glass and pens and books scattered on the floor. Pools of blood were seen on the floor of the open-air cafeteria.

The Britain-based Syrian Observatory for Human Rights confirmed the attack saying many of the wounded were in critical condition.