An increasing number of "foreign elements" including jihadis are now operating in Syria, an independent UN panel confirmed Monday in its first report to say that outside "terrorists" have joined a war spiralling out of control.

The investigative panel appointed by the Human Rights Council says some of these forces are joining armed anti-government groups while others are operating on their own.

"Such elements tend to push anti-government fighters towards more radical positions." the head of the panel, Brazilian diplomat and professor Paulo Sergio Pinheiro, told diplomats. He referred to the foreigners as "terrorists," though the word did not appear in the written report.

Activists say at least 23,000 people have been killed in Syria in the past 18 months.

The panel accused government forces and pro-regime shabiha militia of war crimes and crimes against humanity including murder, summary executions, torture, arbitrary arrests, sexual violence and abuse of children. It also accused anti-government armed groups of war crimes including murder, extrajudicial execution and torture.

Pinheiro said that the human rights situation has "deteriorated to such a degree that it is difficult to describe justly in such a few words. Gross violations of human rights have grown in number, in pace and in scale."

He said the frequency of these "egregious violations" were so enormous that his panel could no longer investigate them all.

"Civilians, many of them children, are bearing the brunt of the spiralling violence," he said.

Syrian authorities have blamed the anti-government uprising that began in March 2011 on a foreign conspiracy and accused some Gulf and Western countries of offering funding and training to the rebels, whom they describe as terrorists.

Syrian UN Ambassador Faysal Khabbaz Hamoui contested the report's overall accuracy and objectivity. But he appeared to agree with Pinheiro on the presence of outside elements, telling the council that "many international parties are working on increasing the crisis in Syria."

Turkey's UN Ambassador Oguz Demiralp told diplomats that "the crisis is spiraling further downward with no end in sight."