DAMASCUS  - An air strike on a rebel town near Damascus killed 13 women and children on Monday, a watchdog said, fuelling international calls for a war crimes probe into the 22-month Syrian conflict.Reports of the civilian deaths came as Human Rights Watch accused President Bashar al-Assad's regime of expanding its use of banned cluster bombs.Monday's air strike on houses in the town of Moadamiyat al-Sham, southwest of Damascus, killed at least eight children and five women, the Syrian Observatory for Human Rights said. "The children, all members of the same clan, were aged between six months and nine years old," said the head of the Britain-based Observatory, Rami Abdel Rahman.State television blamed "terrorists" for the deaths.Also near the capital, two siblings aged six and seven were killed, the Observatory said, while another child was killed in the northern city of Aleppo.The Observatory says that more than 3,500 children have been killed since the Syrian conflict erupted in March last year. The United Nations says more than 60,000 people have died in all. On the diplomatic front, at least 55 governments prepared on Monday to demand that the UN Security Council refer the conflict to the International Criminal Court.The demand was to be made in a letter organised by Switzerland, which has spent seven months collecting signatories.Diplomatic sources said that 55 countries had signed and others could still join even though the initiative has little immediate chance of success.The Security Council is deadlocked over the conflict, with veteo-wielding permanent members Russia and China having blocked three resolutions threatening sanctions against Assad's regime.And with neither being members of The Hague-based ICC court, they would almost certainly reject any new resolution proposing war crimes charges.On Sunday, Russia said Assad's removal from power was not a part of past international agreements on the crisis and so impossible to implement."This is a precondition that is not contained in the Geneva communique (agreed by world powers in June) and which is impossible to implement because it does not depend on anyone," news agencies quoted Foreign Minister Sergei Lavrov as saying.The wrangling comes amid warnings that the conflict, which according to the UN has sent more than 600,000 Syrians fleeing into neighbouring countries, is growing more dangerous for civilians in the face of the regime's expanded use of cluster bombs.New York based Human Rights Watch said Damascus was increasingly resorting to firing rockets containing the sub-munitions, after previously using only aircraft.Syria "is now resorting to a notoriously indiscriminate type of cluster munition that gravely threatens civilian populations," the director of HRW's arms division Steve Goose said in a statement. UN-Arab League peace envoy Lakhdar Brahimi, who last week dismissed peace proposals by Assad as "one-sided", came in for more criticism from the Syrian authorities with government daily Al-Thawra describing him as an "ageing tourist"."If he doesn't have a solution, he'd better leave Syrians alone," the paper said.