United Nations human rights investigators said on Monday that they had drawn up a new secret list of Syrians and units suspected of committing war crimes who should face criminal prosecution some day.

The independent investigators, led by Paulo Pinheiro, said they had gathered "a formidable and extraordinary body of evidence" and urged the UN Security Council to refer the situation in Syria to the International Criminal Court (ICC).

"A second confidential list of individuals and units believed to be responsible for violations is being provided to the High Commissioner of Human Rights," Pinheiro told the UN Human Rights Council in Geneva.

He said there was an "increasing and alarming presence" of Islamist militants in Syria, some joining the rebels and others operating independently. Their presence tended to radicalise the rebels who have also committed crimes, he said.

Western countries are seeking yet another condemnation of Syrian President Bashar Al-Assad's government at the session. "The international community must ensure impunity will not prevail," European Union ambassador Mariangela Zappia said at Monday's debate that was also attended by Syrian envoy Faysal Khabbaz Hamoui.