BEIRUT - Two mortar shells struck a school complex in central Damascus on Tuesday, killing at least 12 people and wounding dozens, state media and a monitoring group said.

The attack in the mainly Shi'ite Muslim district of Shaghour came a day after President Bashar al-Assad nominated himself to stand for re-election, in defiance of calls for him to step aside and allow a political solution to Syria's civil war.

Residents said the Badr el-Din Husseini complex which was hit is attended by both primary and secondary school students.

State news agency SANA said 12 people were killed but the Britain-based Syrian Observatory for Human Rights, which monitors the violence in Syria, put the death count at 13.

Assad's forces have pushed back the mainly Sunni Muslim rebels from many of their strongholds around the capital, but residents say the insurgents have stepped up rocket and mortar attacks against the heart of the city in recent weeks.