“We have 1,800 ISIS Prisoners taken hostage in our final battles to destroy 100% of the Caliphate in Syria. Decisions are now being made as to what to do with these dangerous prisoners,” Trump wrote in a pair of tweets.

He then claimed that “European countries are not helping at all, even though this was very much done for their benefit,” and accused them of “refusing to take back prisoners from their specific countries.”

The Trump administration declared Syrian territory completely free of ISIS control in March, as the president has urged the withdrawal of most U.S. troops there, and is now looking for ways to stabilize the country once it pulls back.

But Trump's missive comes a little over a week after ISIS took credit for a rash of bombings that killed more than 250 Easter worshippers in Sri Lanka, and days after the group released a new video that purportedly shows its leader, Abu Bakr al-Baghdadi, alive on film for the first time in five years.

In February, Trump threatened to release what he said were 800 ISIS prisoners if European allies like Britain, France and Germany refuse to “step up” and take in the fighters to be put on trial.

“The alternative is not a good one in that we will be forced to release them,” he wrote in a tweet then.

“The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go. We do so much, and spend so much - Time for others to step up and do the job that they are so capable of doing," he added.

The Guardian reported this month that Iraq is in negotiations with the U.S. over the issue of where to send detained members of ISIS, and has named a high fee in exchange for agreeing to receive and try them.

In addition to a $10 billion upfront fee, Baghdad has asked for a $1 billion annual fee and has proposed banning humanitarian workers from facilities in the country and requiring no objection to the death penalty, The Guardian reported, conditions that could be nonstarters for countries like France and the U.K.