U.N. investigators are warning of a growing risk of "internationalization" of the conflict in Syria, which they say is on the brink of collapse.

The Independent International Commission of Inquiry on Syria says in a new report Monday that the conflict, which soon enters its sixth year, has become "a multi-sided proxy war steered from abroad by an intricate network of alliances."

In a swipe at the United States, Russia and their allies, the investigators say the international powers and regional countries ostensibly pushing for peace are the same nations that "continue to feed the military escalation."

The report also says war crimes are "rampant" and both government forces and the Islamic State extremist group continue to commit crimes against humanity.