BEIRUT: Shelling of the Yarmuk Palestinian refugee camp in the Syrian capital killed at least 15 civilians, including two children, the Syrian Observatory for Human Rights said on Friday.

The watchdog said the mortar shells slammed into the camp, on the southern outskirts of Damascus, on Thursday night, as President Bashar al-Assad's regime pressed its bid to crush an uprising that erupted almost 17 months ago.

Observatory director Rami Abdel Rahman said the shelling occurred as clashes flared between government troops and opposition fighters in the nearby Damascus neighbourhood of Tadamun.

“We demand an international investigation. We do not know the origin of the shelling,” Abdel Rahman told AFP in Beirut on the telephone.

The Observatory also reported heavy shelling overnight around Houla, a town in the central province of Homs where at least 108 people were massacred at the end of May, triggering international outrage.

More than 20,000 people have been killed in Syria since the anti-Assad revolt began in March 2011, according to the Observatory. There is no way to independently verify this figure, while the UN has stopped keeping count.