OUTSIDE BAGHOUZ, Syria - US-backed Syrian forces fighting the Islamic State group in Syria handed over more than 150 Iraqi members of the group to Iraq, the first batch of several to come, an Iraqi security official said Thursday.

The official said the IS militants were handed over to the Iraqi side late Wednesday, and that they were now in a “safe place” and being investigated. The transfer marks the biggest repatriation from Syria of captured militants so far — an issue that poses a major conundrum for Europeans and other countries whose nationals have been imprisoned as foreign fighters in Syria. The SDF is holding more than 1,000 foreign fighters in prisons it runs in the country’s north, many of them Iraqis and Europeans.

The Kurdish-led Syrian force — and more recently President Donald Trump — have called on these countries to take back their nationals. SDF says it cannot afford to keep the captured foreigners in Syria, but few of their countries want them back.

Earlier this month, Iraqi Prime Minister Abdul-Mahdi said Iraq will take back all Iraqi IS militants in Syria, as well as thousands of their family members. The Iraqi security official, who spoke Thursday on condition of anonymity in line with regulations, said the SDF are holding more than 20,000 Iraqis suspected of IS membership as well as their families in prisons and camps in northern Syria.

An Iraqi intelligence official said among those were around 500 Iraqi IS fighters, adding that these will be transferred back home in batches.