DAMASCUS (AFP) - Mortar fire killed at least 15 students at Damascus University on Thursday, with state media blaming "terrorists" - its term for Syrian rebels - who are increasingly targeting President Bashar al-Assad's seat of power.

The attack came as battles between insurgents and loyalists troops raged in several districts on the edges of the city, and as warplanes targeted rebel enclaves in Irbin and other towns east of Damascus.

"A total of 15 students were killed in a mortar attack launched by terrorists targeting the architecture faculty," university dean Amer Mardini was quoted by the official SANA news agency as saying.

The agency added that six others were wounded by "mortars that targeted the faculty cafeteria". Al-Ikhbariya, a pro-regime television channel, ran footage showing a bloodied patio filled with broken glass and upturned chairs.

The broadcaster also showed doctors treating seriously wounded young people, some of them unconscious.

Syria's national student union immediately denounced the attack.

"The National Students Union condemns the cowardly terrorist attack that targeted the architecture faculty of Damascus University," state television said.

The attack comes as rebels fighting to oust Mr Assad's regime have escalated mortar attacks on central Damascus, including Umayyad Square in the middle of the capital, which houses state television's headquarters.

The Syrian Observatory for Human Rights, a Britain-based monitoring group that relies on sources on the ground for its information, also reported Thursday's attack, although it put the toll at 13 people dead.

Director Rami Abdel Rahman said the recent escalation of mortar attacks on Damascus has instilled terror in the capital.

"Because of the regular mortar attacks on Baramkeh and other areas in Damascus, residents no longer feel safe to walk in the streets or to go about their daily lives," he told AFP.

"The vast majority of victims of recent ones on Damascus have been civilians," he added.

Meanwhile, despite an all-out army attempt to crush the insurgency there, battles raged in several areas on the edges of Damascus, the Observatory said.

Among the afflicted neighbourhoods were Qaboon in the northeast, Yarmuk and Al-Hajar Al-Aswad in the south and Qadam in the southwest.

The air force unleashed new strikes on Irbin, Deir Salman and Nashabiyeh east of Damascus, the Observatory said.

Thursday's violence comes a day after at least 148 people were killed in violence across Syria, the group added.

More than 70,000 people have been killed in Syria's spiralling conflict and millions more have been displaced, the United Nations says.