International calls for a war crimes probe into the 22-month Syrian conflict are growing after a watchdog reported at least 26 children have been killed in the latest violence.
Reports of the child deaths came on Monday as New York-based Human Rights Watch accused President Bashar al-Assad's regime of expanding its use of banned cluster bombs.
At least 57 governments have called on the UN Security Council to refer the Syria conflict to the International Criminal Court for a war crimes investigation.
Switzerland sent a petition requesting the move to the 15-member council, the only body that can refer the case to the ICC. But the Security Council, including its five permanent members, is deeply divided over the conflict.
The signatories included many European governments as well as Libya and Tunisia, which both saw Arab Spring uprisings overthrow longstanding autocratic regimes.
The letter called on the Security Council to refer the Syria conflict for an ICC investigation "without exceptions and irrespective of the alleged perpetrators."
As Syria is not an ICC member, only a Security Council referral could start a war crimes investigation.
The push came as eight children and five women were killed in an air strike on the town of Moadamiyat al-Sham, southwest of Damascus, according to the Syrian Observatory for Human Rights.
"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.
Four other children, including two siblings, were killed in a separate attack, near the capital, the Observatory said. Another eight were killed in the northern province of Aleppo -- five of them in a single air strike.
Six more children died in other flashpoints in the strife-torn country.
The Observatory says more than 3,500 children have been killed since the Syrian conflict erupted in March 2011. The United Nations says more than 60,000 people have died in all.
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In a separate statement Steve Goose, director of HRW's arms division, added: "Syria is escalating and expanding its use of cluster munitions, despite international condemnation of its embrace of this banned weapon.
"It is now resorting to a notoriously indiscriminate type of cluster munition that gravely threatens civilian populations."
The rights group said other governments should sign up to the Swiss-led initiative.
But Russia and China, both veto-wielding permanent members of the Security Council, have refused to sign the petition.
Russia's Deputy Minister of Foreign Affairs, Mikhail Bogdanov, said on Monday that the five permanent members of the Security Council would meet to discuss the Syrian crisis towards the end of January.
Bogdanov also told Interfax news agency that the UN was looking at ways of sending a new observer mission to Syria.
Aid agency Medecins Sans Frontieres (Doctors Without Borders) meanwhile condemned a Sunday air strike on the northern town of Aazaz in Aleppo province, near the border with Turkey, that wounded 99 people.
"The attack... was particularly devastating as it came just two weeks after air strikes hit the city's health facilities, making it almost impossible for medical staff to cope with an emergency on this scale," MSF said.
"In Syria the massacre has to stop," EU President Herman Van Rompuy said Monday after talks in Cairo with Arab League chief Nabil al-Arabi.
"According to the recent UN assessment, more than 60,000 people have already died. This is appalling and unacceptable."
Van Rompuy reiterated the EU's position that Assad should step aside in favour of a democratic transition.
The head of the World Food Programme Ertharin Cousin said Syrian refugees urgently needed more aid.
Inside Syria alone, they were providing help to approximately 1.5 million people at a cost of $25 million (19 million euros) per month, she said. Cousin is due to visit camps in Turkey on Tuesday.
The UN refugee agency UNHCR reported Monday that the number of Syrian refugees in neighbouring Turkey had grown to more than 153,000 people.