Washington wants European allies to take back hundreds of IS fighters from their countries who were captured in recent months in Syria.

The alternative is to hold them long-term in camps or prisons in Syria or Iraq, but that would need financing.

"We have assured once more that we will take care individually of every single case," von der Leyen told reporters after meeting Shanahan, the acting secretary of defense.

"But we were both also convinced of the fact that the challenge consists mostly in collecting the evidence in the region as regards to the terrorist activity of the terrorist fighters," she added.

Some European countries are refusing to repatriate and put on trial their nationals given the absence of any investigation on the ground in Syria showing what they might have done with Islamic State.

In addition, the fighters are being held by the Syrian Democratic Forces, who do not have the status of a sovereign government.

But Washington fears that if they are not repatriated, the fighters could rejoin jihadist operations. The US wants the Europeans to finance the building of detention centers, which could be in Iraq, according to the Pentagon.

Baghdad has said it could put on trial the alleged jihadists, but that it would need funding to support that.

That would solve the problem for their home countries, but human rights groups are uncomfortable with that path.

Von der Leyen meanwhile expressed satisfaction with the assurances she received that US forces will maintain a presence in Syria "with the required number of troops," after Trump said in December that the Pentagon would pull out following the defeat of Islamic State.

"The next weeks will show how everybody can bear their fair share of the burden," she said.