BEIRUT — Mortars rained down on a crowded marketplace in a Palestinian refugee camp in the Syrian capital, killing 21 people as regime forces and rebels clashed on the southern outskirts of Damascus, activists said Friday.

The Britain-based Syria Observatory for Human Rights, which reported the deaths, said the shells hit Yarmouk camp Thursday as shoppers were buying food for the evening meal. The activists with the group would not speculate on who was firing.

"We don't know where the mortars came from, whether they were from the Syrian regime or not the Syrian regime," said Rami Abdul Rahman, director of the Observatory. He added they could also have been strays from the fighting in nearby Tadamon neighborhood.