WASHINGTON — United States-backed Kurdish militias in northern Syria are detaining hundreds of Islamic State (IS) fighters and family members in makeshift camps, raising fears among US military officials of potentially creating a breeding ground for extremists — repeating a key security mistake of the Iraq War.

Despite its concerns, the Trump administration has largely taken a hands-off approach toward the detainees, who come from more than 30 countries and were captured or surrendered after last year’s collapse of Raqqa, IS’ self-proclaimed capital.

Unlike suspected IS militants seized in neighbouring Iraq, largely from the northern city of Mosul and surrounding areas, the detainees being held in the Kurdish region of Syria fall into a legal gray area and face an uncertain long-term fate.

Kurdish authorities are meting out justice in ad hoc courts, but the region is still part of Syria, and Kurdish control is not internationally recognised.

Some countries like Russia have signaled that they will repatriate their fighters, but many other nations are refusing.

The detention of the IS fighters is just one issue the US is grappling with in its partnership with Syrian Kurds.

US troops have fought alongside Kurdish-led militias on battlegrounds against the IS in northeast Syria, and the Pentagon is showing no sign of backing away from them.

At the same time, however, Washington is trying to tamp down escalating tensions in Syria between the Kurdish militias and Turkey, a longtime North Atlantic Treaty Organization (Nato) ally that views the Kurds as a terrorist threat.

US Special Operations troops advising the Kurdish-led militia known as the Syrian Democratic Forces are cataloging fingerprints and other so-called biometrics of many of the estimated 200 to 300 detainees in at least three camps near Raqqa.

The US forces also are interrogating the detainees to learn more about foreign fighter networks and threats to their home countries.

But US military officials see parallels with the Iraq War, in which militants, including Abu Bakr Baghdadi, the current head of IS, were held for years at Camp Bucca, a sprawling American detention facility on the Kuwaiti border where they became more radicalised.

“Clearly, we’ve seen what happens when you have a group of highly trained, terrorist fighters held in detention for a long time,” Colonel Ryan Dillon, a spokesman for the US-led military coalition in Baghdad, said in a telephone interview. “We don’t want to see that, and it’s something we’re addressing.”

If so, the initial efforts appear tentative.

The Trump administration has quietly established an interagency working group, including officials from the State, Defence and Justice departments, to help the Syrian Kurds deal with the problem. But US officials seem to want to wash their hands of the growing problem, despite the potential security and humanitarian risks.

“Foreigners who take up arms in Iraq and Syria and are subsequently captured in the field are not necessarily turned over to the coalition, nor is there any requirement for the coalition to be notified,” the US-led military coalition in Baghdad said in an email.

Mr Marc Raimondi, a spokesman for the White House’s National Security Council, referred all questions to the State Department. When asked for comment, the State Department said in an email: “We are working to address issues related to IS fighters detained in other countries. This includes discussions with foreign partners.”

International relief organisations are seeking more information about conditions in the camps over the long term, particularly on widows and children of suspected fighters.

“In Syria, we are involved in ongoing discussions with regard to detention access,” said Mr Marc Kilstein, a spokesman for the International Committee for the Red Cross in Washington, who declined to provide details to avoid jeopardising its work in the region.

A Kurdish journalist, Mr Arin Sheikhmus, 30, said he had visited three camps .

Mr Mostapha Bali, a spokesman for the Syrian Democratic Forces, said conditions in the camps met international standards, especially for women and children.

He said Kurdish authorities were sorting out which detainees were actual combatants and which were local civilians pressed by IS into administrative or medical jobs, and could be safely released.

Even as Kurdish authorities, US officials and relief organisations seek to address the IS prisoners in custody, broader concerns remain about those fighters still at large.

Some 40,000 fighters from more than 120 countries poured into the battles in Syria and Iraq during the past four years, US and other Western officials say.

While thousands died on the battlefield, officials say many thousands more probably survived to slip away to conflicts in Libya, Yemen or the Philippines, or have gone into hiding in countries like Turkey.

Of the more than 5,000 Europeans who joined those ranks, as many as 1,500 have returned home, including many women and children, and most of the rest are dead or still fighting, according to Mr Gilles de Kerchove, the European Union’s top counter-terrorism official.

European intelligence services, along with Interpol, have created major new databases of suspected foreign fighters. European spy agencies and Europol have also created counterterrorism hubs in the Netherlands for sharing information and mapping out strategy.

The United Nations Security Council last month unanimously approved a resolution requiring all nations to collect airline passenger data, maintain watch lists of known and suspected terrorists, and collect biometrics, such as fingerprints, to help spot foreign fighters if they attempt to board planes.

In the field, US warplanes and Kurdish-led ground forces are hunting for the roughly 1,000 remaining IS fighters hiding along the Euphrates River valley near the border with Syria and Iraq. US counterterrorism officials believe Baghdadi is most likely hiding in these Sunni border areas straddling the two countries.

In a Senate hearing this month, Mr David Satterfield, the State Department’s acting top Middle East diplomat, warned lawmakers that top IS operatives still posed a serious threat. “Many of its core leadership and cadre avoided the fight,” he said. “They remain present and they remain coherent.” THE NEW YORK TIMES