U.S. President Donald Trump has slammed European countries for not making an effort to take back Daesh militants being held in Syria.

"We have 1,800 ISIS [Daesh] prisoners taken hostage in our final battles to destroy 100 percent of the Caliphate in Syria. Decisions are now being made as to what to do with these dangerous prisoners," Trump wrote on Twitter.

"European countries are not helping at all, even though this was very much done for their benefit. They are refusing to take back prisoners from their specific countries. Not good!" he added.

Trump made a similar call in February when he threatened to release the prisoners if European countries did not accept to take back their citizens. However, many European countries remained indifferent to the issue. "The U.S. is asking Britain, France, Germany and other European allies to take back over 800 [Daesh] fighters that we captured in Syria and put them on trial. The caliphate is ready to fall. The alternative is not a good one in that we will be forced to release them," Trump wrote on Twitter on Feb. 17.

Following the U.S. President's demand, German Foreign Minister Heiko Maas said German laws give its nationals the right to re-entry, but the country would discuss the demand with Washington as well as its European partners, notably France and Britain.

French Justice Minister Nicole Belloubet said on the issue that the country will take back French militants on a case-by-case basis.

The PKK's Syrian affiliate, the People's Protection Units (YPG), which is backed by the U.S., is holding captive a large number of Daesh militants, along with their families, and often uses them to secure its support from the U.S. and European countries.

Last December, after Trump announced Washington's withdrawal from Syria, a YPG official said the terrorist group was planning to release Daesh prisoners, The New York Times reported.

The group released nearly 300 Daesh militants in March. "They had lost their way... violated the traditions of the Syrian society and the law, and some of them had been deceived... but they remain our Syrian children," a YPG statement said after release, emphasizing that releasing Daesh militants was a gesture of "cooperation, fraternity and clemency."