Syria's US-backed Kurdish forces, which are battling Islamic State group jihadists in their last redoubt in eastern Syria, hold hundreds of suspected foreign IS fighters and the calls for their reluctant home countries to take them back have grown in urgency.

"We must be able to ensure that prosecution is possible," Defence Minister Ursula von der Leyen told Bild daily.

Underlining the difficulties however of putting the ex-fighters on trial, the minister noted that there is "no government in Syria with which we have a sensible relationship".

President Bashar "Assad cannot be our counterpart, the Syrian-democratic forces are not a unity government," she added, stressing that proof and witness statements needed to be secured in Syria if the militants are to be put on trial.

Foreign Minister Heiko Maas said separately that a return could only be possible if "we can guarantee that these people can be immediately sent here to appear in court and that they will be detained".

For this, "we need judicial information, and this is not yet the case," Maas told ARD television late Sunday. Under such conditions a repatriation would be "extremely difficult to achieve".

Berlin wants to "consult with France and Britain... over how to proceed," he said.

The subject is to be raised on Monday at a meeting of European foreign ministers called to discuss among other issues "the situation in Syria, in particular the recent developments on the ground," according to an agenda for the talks.

Trump on Sunday called on his European allies to take back alleged jihadists captured in Syria.

IS imposed a self-declared caliphate across parts of Syria and neighbouring Iraq from 2014, but has since lost all of it except a tiny patch of less than half a square kilometre near the Iraqi border.

After years of fighting IS, the Kurdish-led Syrian Democratic Forces (SDF) hold hundreds of foreigners accused of fighting for the group, as well as their wives and children.

Syria's Kurds have repeatedly called for their countries of origin to take them back, but these nations have been reluctant.

"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," Trump said in a tweet, using another acronym for IS.

After initial reluctance, Paris appears ready to consider the return of its nationals.

In Belgium, Justice Minister Koen Geens called for a "European solution" on Sunday, calling for "calm reflection and looking at what would be the least security risks".