BEIRUT (AP) — 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 today.

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 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.

The state news agency blamed the bombardment on “terrorist mercenaries” — a term the government uses for rebel fighters — and said they had been chased away by security forces.

Government troops, however, have in the past attacked the camp, home to nearly 150,000 Palestinians and their descendants driven from their homes by the war surrounding Israel’s 1948 creation. Palestinian refugees in Syria have tried to stay out of the 17-month old uprising but with Yarmouk nestled among neighborhoods sympathetic to the rebels, its residents were eventually drawn into the fighting.

Yarmouk’s younger inhabitants have also been moved by the Arab Spring’s calls for greater freedoms and have joined protests against President Bashar Assad’s regime— and have died during demonstrations when Syrian troops fired on them.

With the civil war in Syria getting increasingly vicious, chances for a diplomatic solution to the conflict were fading after the resignation Thursday of Kofi Annan, the U.N.-Arab League envoy to Syria. Annan cited divisions within the Security Council preventing a united approach to stop the fighting.

The U.N. General Assembly was preparing to vote today on a new Arab-sponsored resolution condemning Syria’s use of heavy weapons to crush the uprising that has killed an estimated 19,000 people since it began on March 2011.

The resolution — which like all General Assembly resolutions is unenforceable — is expected to denounce Syria for unleashing tanks, artillery, helicopters and warplanes on the people of Aleppo and Damascus, and demand that the Assad regime keep its chemical and biological weapons warehoused and under strict control.

The attack on the Palestinian camp came as clashes raged overnight between rebels and government forces in the nearby Damascus suburb of Tadamon, which was also bombed by army Thursday, sending plumes of black smoke over the city.

The Observatory also reported shelling of the southwestern suburb of Jdaidat Artouz, where dozens of bodies were found after government forces swept through on Wednesday.

Syria’s civil war, which had raged across much of the country, came to the capital and northeastern city of Aleppo, Syria’s main commercial hub in July.

A rebel assault and revolt in Damascus two weeks ago was vigorously crushed by government forces, but pockets of resistance and sympathetic neighborhoods remain. Sporadic clashes and shelling also continue in Aleppo.