(Reuters) - The fate of foreigners who joined Islamic State has become an increasingly urgent issue as U.S.-backed fighters prepare to capture the militant group’s last stronghold in eastern Syria.

The Kurdish-led, U.S.-backed Syrian Democratic Forces say they are holding 800 foreign fighters, with 700 of their wives and 1,500 of their children living separately in camps. Dozens more militants and family members have been arriving daily.

U.S. President Donald Trump said on Sunday European countries must do more to take back their citizens or they will be released. The SDF says it has no intention of freeing them, but they could escape in future if the area is attacked.

European countries say they are working on how to deal with their citizens who want to come home, including investigating and prosecuting those guilty of crimes.

But they say this is made more complicated because diplomats cannot operate in territory held by the SDF, which is not internationally recognized, and they worry that the Trump administration’s decision to withdraw its 2,000-strong force from Syria means there will not be enough time to respond.

According to European police agency Europol, some 5,000 Europeans -- most from Britain, France, Germany and Belgium -- went to fight in Syria and Iraq, of whom some 1,500 have returned.

Following is a breakdown by country of Europeans believed to have traveled to Syria or Iraq.

BRITAIN:

British officials estimate around 900 people from the United Kingdom went to the Syria region. Around a fifth have died and about half have returned, so around 360 remain in the region. Officials noted that not all of those who went to the Syria region were extremists: some went with genuine charitable intentions. Of those who went to fight, not all joined Islamic State, though most of those left in the region are thought to represent militant elements.

The issue has gained greater attention in recent days as news organizations have run interviews with Shamima Begum, a pregnant 19-year-old who left London to become an Islamic State bride as a schoolgirl.

BELGIUM:

The Belgian threat assessment center OCAD/OCAM says 442 Belgians left for Syria or Iraq, of whom 130 have returned and 142 have been killed. Six Belgian men, 17 women and 32 children are being held in Syria, and two men are jailed in Iraq.

DENMARK:

The latest data available from Denmark’s security and intelligence service dates to January 2018. The authorities believe at least 150 people have left Denmark for Syria or Iraq since the summer of 2012 to join militant Islamist groups. More than a third have returned to Denmark and at least a quarter are presumed dead. Roughly one fifth are still in the conflict zone, with the rest located in third countries.

FRANCE:

French officials and the International Center for the Study of Radicalisation and Political Violence estimate that 800-1,500 people have traveled from France to join Islamic State. French officials estimate that around 300 have been killed and a similar number have returned to France.

Of those that remain in Syria and Iraq, diplomats and military officials said two weeks ago that Kurdish-led forces were holding about 150 French citizens in northeastern Syria. An estimated 100 French citizens are believed to be left in the last pocket of Islamic State fighters, although it is not clear if that includes families and children. A further 150 French fighters and children are in Idlib, French officials have said.

GERMANY:

According to the German authorities, around 1050 people from Germany have traveled to Syria and Iraq since 2013, around one third of whom have returned. Around 270 women and children from Germany are now in Syria and Iraq. Around 75 percent of the children are under the age of three years old and believed to have been born in the conflict zone. The Interior Ministry estimates the number of Germans in the custody of the Kurdish-led forces in northern Syria as in the high double digits.

NETHERLANDS:

The Dutch intelligence agency AIVD says about 315 people left the Netherlands for Syria or Iraq with the intention of joining the militants, about two thirds of them men and one third women. About 55 are believed to have returned, 85 are believed to have died, 135 are still in the conflict area and 40 are either in third countries or in detention in the region.

Sources told Reuters last year that as many as 175 children with the right to Dutch citizenship were believed to be in the conflict zone.