(CNN) In a series of tweets, President Donald Trump has said European countries should take back and put on trial hundreds of ISIS fighters who have been captured in Syria.

"Time for others to step up and do the job that they are so capable of doing," the US President tweeted late Saturday night.

He warned that as ISIS is "ready to fall," more than 800 prisoners could make their way to Europe.

"The alternative is not a good one in that we will be forced to release them," Trump tweeted.

"The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go."

The United States is asking Britain, France, Germany and other European allies to take back over 800 ISIS 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........

....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. We are pulling back after 100% Caliphate victory!

Trump's comments come as the area that ISIS now controls has shrunk to just half a square kilometer , a commander with the US-backed Syrian Democratic Forces (SDF) said on Saturday.

The SDF has launched an offensive to oust ISIS from its last enclave in Syria, in the small town of Baghouz Al-Fawqani in the country's east.

At its height, the group controlled an area the size of Great Britain and ruled over 10 million people.

The top US general in Iraq, Lt. Gen. Paul LaCamera, responded to a question by CNN's Barbara Starr on Sunday about the issue of the detainees and said, "The numbers are not negligible. And what happens to them is, as you stated, that's the big question."

"Now here's the real challenge: if you joined ISIS, you handed over your passport, you handed over your ID card, you handed over your birth certificate and that was destroyed and you were issued into ISIS - so the real question is, it's not just the prisoners, the detainees, it's what are we going to do with these stateless people?" LaCamera told reporters.

LaCamera said the issue is "not in the military's hands. This is really in diplomatic channels."

"It's not just the detainees - it is those that have been displaced by ISIS or don't have an identity," LaCamera said. "How are we going to deal with them? You know, will the Iraqis take them? And again, if you're a refugee, you can't force them back, right?"

UK 'will not hesitate' to prevent fighters' return

On Friday UK Home Secretary Sajid Javid said Britain will "not hestitate" to prevent the return of ISIS fighters to the UK, but those who make it back will be investigated and potentially prosecuted.

Downing Street referred CNN to Javid's Friday statement when asked about Trump's tweets.

"We have a range of tough measures to stop people who pose a serious threat from returning to the UK, including depriving them of their British citizenship or excluding them from the UK," Javid said.

"We closely monitor any returnees who pose a risk and new terror legislation passed this week allows us to make traveling to certain designated regions an offense. We also support communities in their efforts to prevent radicalization."

The Home Secretary added that it was his priority to "ensure the safety and security of the UK" and that he won't "let anything jeopardize this."

Shamima Begum traveled from London to Syria with two of her classmates when she was 15 to join the caliphate. Now 19, she told The Times that she had no regrets about going to Syria but that she wanted to come home "to have my child." On Saturday, her family's lawyer announced Begum had given birth to her third child.

Meanwhile, the German Interior Ministry said Sunday that citizens suspected of fighting for ISIS have the right to return to Germany.

"In principle, all German citizens, including those suspected of having fought for the so-called IS, have the right to return to Germany," spokeswoman Susanna Hartung told CNN in a statement. She added that an individual risk assessment is made for each person.

About 1,050 people are known by Germany security authorities to have left for Syria and Iraq since 2013 "in order to join the side of terrorist groups," the Interior Ministry said. One third, it added, have already returned to Germany.

"Wherever possible, the German authorities are trying to deradicalize returnees. Support services, such as deradicalization measures, but also socio-educational and psychological support, are particularly important for returning children who may be severely traumatized and for whom criminal proceedings are out of the question," the statement said.

The US continues to face criticism for the plan , a move that shocked allies, appalled US lawmakers and triggered resignations -- including that of Defense Secretary James Mattis.

Correction: This story has been updated to correct the amount of territory that ISIS holds in Syria. It controls about half a square kilometer.