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. Shells also rained down on rebel positions in Aleppo Friday ahead of a UN vote to deplore both the Syrian regime's use of heavy arms and world powers for failing to agree on steps to end the conflict. 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. Government troops, however, have in the past attacked the camp, home to nearly 150,000 Palestinians and their descendants driven from their homes. 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. 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 the army Thursday, sending plumes of black smoke over the city. Despite the violence, new weekly anti-regime protests were held across Syria in solidarity with the eastern province of Deir Ezzor, which troops have also pounded for weeks. Activists were still counting the toll from Thursday, one of the bloodiest days in the uprising, when UN envoy Kofi Annan quit as international envoy for Syria complaining his peace plan never received the backing it deserved. The Observatory said more than 179 people were killed - 110 civilians including 14 children, 43 soldiers and 26 rebels. Meanwhile, three major Russian news agencies reported that Russia is sending three warships carrying some 360 marines total to the Syrian port of Tartus. But the Russian Defense Ministry denied reports. – Agencies