President Donald Trump speaks during a meeting in the Cabinet Room of the White House in Washington, D.C., on Tuesday, Feb. 12, 2019.

The United States is asking its European allies to "take back over 800" ISIS fighters that have been captured in Syria and put them on trial, President Donald Trump tweeted late on Saturday.

"The Caliphate is ready to fall," he said on the social media site. "The alternative is not a good one in that we will be forced to release them..."

U.S.-backed fighters in Syria are poised to capture the so-called Islamic State's last, tiny enclave on the Euphrates, Reuters reported Saturday citing the battle commander, bringing its self-declared caliphate to the brink of total defeat.

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Jiya Furat, the commander of the assault by the Syrian Democratic Forces (SDF), said they had cornered the remaining militants in a neighborhood of Baghouz village near the Iraqi border. Trump has sworn to pull U.S. forces from Syria after ISIS's territorial defeat, raising questions over the fate of Washington's Kurdish allies and Turkish involvement in northeast Syria.

Trump said Saturday that the U.S. is asking Britain, France, Germany and other European allies to take back over 800 ISIS fighters that had been captured and put them on trial.

The issue has come to the fore in Britain, in particular, after a recent interview with Shamima Begum — a Briton who traveled to Syria when she was 15 has been found in a refugee camp in northern Syria.

Now 19, Begum is also heavily pregnant. In the interview carried out in the camp, Begum said she did not regret joining ISIS. The U.K. government has taken a hard line, with the interior minister saying he "will not hesitate" to prevent the return of Britons who join the terror group ISIS.