AN air strike on a rebel town near Damascus has killed 13 women and children, fuelling growing 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 deployment of banned cluster bombs.

Monday's air strike on several 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 reported.

"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.

The Observatory says more than 3500 children have been killed since the Syrian conflict erupted 22 months ago. The United Nations says overall more than 60,000 people have died.

On the diplomatic front, at least 55 countries prepared on Monday to demand the UN security council refer the Syria 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 55 countries had signed and others could still join even though the initiative has little immediate chance of success.

The Security Council is locked in a crippling impasse over the Syria conflict, with permanent members Russia and China having vetoed three resolutions threatening sanctions against Assad.

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 hence 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 the conflict, which according to the UN has sent more than 600,000 Syrians fleeing into neighbouring countries, is growing more dangerous for civilians due to the regime expanding its 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 to spread the weapons.