U.S.-backed forces in Syria are holding more than 2,000 suspected Islamic State fighters, U.S. defense officials said, at least double previous estimates and an obstacle to Trump administration plans to withdraw American forces from Syria.

The new estimate compounds the challenge of relocating the captured Islamic State fighters to their home countries and makes it harder for the U.S.-led coalition to wind down operations in Syria, even after the extremists no longer hold territory.

The development puts a greater burden on the U.S., which now may have to help U.S.-allied forces hold the captured Islamic State fighters.

The U.S. military estimated last fall that the American-supported Syrian Democratic Forces, or SDF, was holding 800 foreign fighters from more than 50 countries. The number has increased by hundreds in the past two weeks, American defense officials said, as SDF members reclaimed territory from Islamic State and taken suspected fighters into custody.

In addition, defense officials cited another group of at least 1,000 suspected Syrian and Iraqi extremist fighters who are in detention in Syria, a figure not previously disclosed. The actual number of those fighters could be even higher, officials acknowledged.