US President Donald Trump today threatened to release Islamic State fighters captured by Syria's Democratic Forces (SDF) in Syria on the European border if France, Germany and other European countries do not take back their nationals.

"I defeated the Caliphate, and now we have thousands of prisoners of war and ISIS fighters," Trump told reporters as he met Australian Prime Minister Scott Morrison at the White House, referring to foreign fighters in the group being held in Syria.

"We ask the countries that came from them in Europe to recover the prisoners of war," he said.

"So far I have refused," he said, referring specifically to France and Germany.

In a threatening tone, Trump told the Europeans: "In the end I will say I'm sorry, but either you get them back or we'll bring them back to your borders."

Trump said he would do so "because the United States will not imprison thousands of people who have been captured at Guantanamo and will not keep them in prison for 50 years" because it will cost "billions and billions of dollars."

"We have provided a great service to the Europeans, if they refuse to take them back we will probably have to send them to the border and they will have to take their families back," he said.

This is not the first time that President Trump has threatened Europeans.In September 2018, he threatened to transfer European jihadists detained in Syria to their home countries and release them if European countries did not return them themselves.