There has been a doubling of volunteers in just the past 12 months which gives stark evidence that an international effort to tighten borders, share intelligence and enforce anti-terrorism laws is not diminishing the ranks of new militant fighters, the New York Times reported.
Among those who have entered or tried to enter the conflict in Iraq or Syria are more than 250 Americans, up from about 100 a year ago, the report said citing intelligence and law enforcement officials.
A year ago, the same officials estimated that flow to be about 15,000 combatants from 80 countries, mostly to join the Islamic State.
That grim appraisal coincides with the scheduled release on Tuesday of a six-month, bipartisan congressional investigation into terrorist and foreign fighter travel, which concludes that "despite concerted efforts to stem the flow" the US has largely failed to stop Americans from travelling overseas to join jihadists.
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Despite Pentagon reports that coalition strikes have killed about 10,000 Islamic State fighters, the group continues to replenish its ranks, drawing an average of about 1,000 fighters a month, the report said.
The government several months ago last publicly assessed the flow at "more than 25,000", including at least 4,500 from the West.
In Britain, more than 750 people have travelled to take part in terrorist-related activity in Syria and Iraq, up from about 500 a year ago. About half of those have returned home, raising fears that they could carry out attacks on British soil.