The number of foreign fighters in Syria is growing, some operating independently and some joining anti-government forces which they are helping to make more radical, U.N. human rights investigators said Monday.

The panel described an escalating level of violence, saying human rights violations are so egregious and so numerous that it is no longer able to investigate all of the cases it uncovers.

The group of independent experts, tasked by the U.N. Human Rights Council to investigate violations in Syria, spoke to the council Monday about developments since it issued a report in August accusing both sides in the civil war of war crimes.

"Gross violations of human rights have grown in number, in pace and in scale," said Brazilian diplomat and professor Paulo Sergio Pinheiro. "Civilians, many of them children, are bearing the brunt of the spiraling violence."

Pinheiro said the panel also is increasingly worried about the increasing number of "foreign elements."

"Such elements tend to push anti-government fighters towards more radical positions," Pinheiro said.

Syria's U.N. ambassador, Faysal Khabbaz Hamoui, contested the report's accuracy and objectivity.