(Updates death toll, adds bus station taken, United Nations pulls out)
By Oliver Holmes
BEIRUT, March 28 (Reuters) - Fifteen Syrian students were killed when rebel mortar bombs hit a Damascus University canteen on Thursday, state-run news agency SANA said, as attacks intensify in the centre of the capital.
A bastion for President Bashar al-Assad's forces, the city is a crucial prize in a two-year-old uprising that has developed into a war in which more than 70,000 people have been killed.
Insurgents trying to end four decades of Assad family rule have formed a semi-circle around the capital and have intensified attacks from positions on the outskirts this week to try to push towards strategic areas and break into the centre.
SANA said the mortar rounds had landed in a canteen at the College of Architecture in Baramkeh, a central district near several government buildings, including the Defence Ministry, the headquarters for state media and Assad's official residence.
Pro-government Al-Ikhbariya television showed images of doctors trying to resuscitate at least two young men and blood splattered on the floor of what appeared to be an outdoor canteen. One young woman was shown walking in a hospital and bleeding heavily from her face.
SANA quoted the president of Damascus University as saying the death toll, initially put at 12, had risen to 15 in what state and pro-government media called a terrorist attack.
The United Nations said on Monday it would withdraw about half its international staff from Damascus after a mortar bomb landed near their hotel.
The Syrian military has responded to rebel attacks with artillery shelling and air strikes on suburbs where rebels are entrenched among thousands of civilians trapped in crossfire.
On Thursday, opposition activists said rebels had taken the main bus station in northeastern Damascus. They provided footage of fighters walking around a deserted area and stamping on a framed picture of Assad. (http://link.reuters.com/peg96t)
Government reporting restrictions make it difficult to verify such accounts independently. (Additional reporting by Reuters TV; Editing by Alistair Lyon)