United States intelligence fears that as many as 30,000 foreign fighters have travelled to Iraq and Syria since 2011, many of them to join the Islamic State (IS) group.
The number reportedly includes many westerners, among them perhaps 250 Americans, and represents a doubling of last year’s US assessment.
The New York Times report cites anonymous “intelligence and law enforcement officials” in its report.
Moreover, a congressional report due out this week was expected to suggest that a year of US-led air strikes has not slowed recruitment.
Meanwhile, US president Barack Obama will chair an international summit this weekend on the sidelines of the United Nations General Assembly for leaders fighting the IS group and “violent extremism.”
US and Western diplomats are scrambling to cobble together a diplomatic strategy to end the war in Syria after the latest humiliating blow to their military plan.
To that end, US Secretary of State John Kerry and his European counterparts reached out to the West’s traditional foe, Iran, at the General Assembly in New York this weekend.
Iran and Russia back the Syrian president, Bashar al-Assad, whom Washington sees as the instigator of a civil war that has left half his country in the hands of the Islamic State group.
Unwilling to countenance a peace process that would leave Mr Assad in power, the US has backed small “moderate” rebel groups.
US strategy shredded
But that strategy appeared to be in tatters on Saturday after the Pentagon admitted the latest US-trained fighters to cross into Syria had given a quarter of their gear to al-Qaeda.
A previous 54-strong group that crossed into northern Syria earlier this year was attacked by al-Qaeda’s local franchise, the Al-Nusra Front, and fell apart, leaving only four or five guerrillas active.
