WASHINGTON (WASHINGTON POST) - A dispute between the United States and Britain over what to do with Islamic State in Iraq and Syria (ISIS) foreign fighters captured on the battlefield is demonstrating the complexities arising in the aftermath of a years-long war to oust the extremist group from its self-declared caliphate in Iraq and Syria.

The latest flashpoint involves two accused ISIS militants who have been held by coalition-backed forces in Syria since mid-January.

The men grew up in Britain and fled to Syria to join a four-person militant cell that became known as the Beatles, owing to the British accents of its members. The cell rose to infamy with the 2014 beheading of American journalists James Foley and Steven Sotloff.

The fate of the two ISIS militants is part of a bigger dilemma for the United States and its coalition partners now that the extremist group has lost nearly all of the territory it once held.

Hundreds of alleged ISIS fighters have been captured on the battlefield, but in many cases, where they should face justice has not yet been determined.

US diplomats and military officers are pushing Britain to accept the two men and bring them to justice - part of an effort by Washington to establish the principle that all foreign fighters captured on the battlefield should be returned to their countries of origin for trial.

Both men are currently being held by the Syrian Democratic Forces (SDF), a Kurdish-dominated group and main US partner in Syria.

Defence Secretary James Mattis said on Tuesday that a final agreement with Britain had not yet been reached.

He called on countries to take responsibility for fighters who joined the ISIS from their soil.

"How they carry out that responsibility, there's a dozen different diplomatic, legal or whatever ways, I suppose," General Mattis said during a trip to Europe. "But the bottom line is, we don't want them going back on the street."

British Defence Secretary Gavin Williamson told the British newspaper the Sun that the men, El Shafee Elsheikh and Alexandra Kotey, turned their back on Britain and should never set foot in the country again.

Apart from leaving British territory to join ISIS, the Beatles cell has also been implicated in the detention and execution of British citizens, including aid worker David Haines.

The State Department has said that if Britain refuses to accept the ISIS fighters, they could end up imprisoned at the military detention facility in Guantanamo Bay, Cuba, which President Donald Trump has promised to keep open indefinitely.

"We are working with coalition partners to determine what to do with ISIS fighters held by the SDF," said Mr Steven Goldstein, the State Department's undersecretary for public diplomacy and public affairs. "One possibility is that former British citizens return to the UK. Another option that we're looking at is to place these terrorist fighters in Guantanamo Bay."

European officials would likely object strenuously to the detention of current or former European citizens at Guantanamo.

Despite Mr Trump's calls to expand the facility, which still houses 41 inmates, officials across the US government are reluctant to do so for legal and diplomatic reasons.

Meanwhile, career Justice Department prosecutors and FBI agents are working feverishly to prepare a case that could bring the Beatles militants to the United States to face trial in federal court, most likely on charges of kidnapping, torturing, and killing American hostages, according to people familiar with the discussions.

By seeking the pair's detention at Guantanamo or a death sentence in civilian court, the United States would risk imperiling any future cases in which Washington seeks to extradite European terrorism suspects.

So far, federal prosecutors are approaching the case as if the militants were apprehended in Europe, meaning they would face neither possibility, the people familiar with the discussions said.

Attorney General Jeff Sessions, who has been a vocal advocate for housing new detainees at Guantanamo, has so far refrained from taking a position on the case, officials said.

The White House official coordinating the decision-making process, Mr Thomas Bossert, favours sending the two men to Guantanamo. But Mr Bossert, who is Mr Trump's top counter-terrorism adviser, also has instructed officials to consider all options.

"There's no rush on the two," said one US national security official familiar with the case. "It's not like there's a gun to our head. We're going to take the time necessary to come up with the right resolution."

Ms Diane Foley, the mother of one of the murdered American hostages, said US law enforcement authorities have not contacted the families about the next step.

"We have made clear what we want to happen," she said. "We want (the two men) to have the opposite of what they gave our children: a fair and public trial."

Ms Foley said she did not favour sending them to Guantanamo, because she thought such a step would "bury the truth".

Some US officials are concerned that the lack of clarity over what to do with ISIS detainees could further exacerbate tensions over how the US government handles cases of Americans taken hostage by overseas terror groups.

During the Obama administration, some families of hostages bitterly criticised the government's approach to such cases, saying US efforts were disorganised and ineffective in rescuing their loved ones.

The two ISIS suspects in question grew up in west London. Elsheikh was born in Sudan. Kotey, who has a Ghanaian and Greek Cypriot background, was born in London. The State Department designated both men as foreign terrorists last year.

The ringleader of the so-called Beatles cell, Mohammed Emwazi, or "Jihadi John", was killed in a 2015 drone strike in Syria. The fourth member, Aine Davis, is being detained in Turkey.

Most of the tension over battlefield detainees has focused on Syria, where non-state groups such as the SDF are holding hundreds of people and lack any official authority to negotiate with foreign governments.

Fighters detained on the battlefield in Iraq, meanwhile, can be processed through the Iraqi justice system, but human rights advocates have highlighted legal shortcomings and due process violations in trials of ISIS suspects there.

The US military is helping the local forces identify the detainees in the hope of repatriating any foreign fighters to their countries of origin for trial.

But the effort, which remains in its early stages, has so far proved unsuccessfully, in part because of the legal concerns these countries have about accepting the combatants.

Many European countries, apart from Britain, also prefer to see the fighters tried in the places where they were fighting, in part to avoid a large influx of tricky cases that would stress domestic criminal justice systems.

In their countries of origin, captives could be prosecuted for crimes they committed before travelling to Syria and Iraq, or for supporting efforts to perpetrate terrorist attacks against their homeland while in Syria and Iraq, Air Force General Paul J. Selva, vice chairman of the Joint Chiefs of Staff, said in late January.

"If the home governments are willing to take these individuals back for the purposes of prosecution, then that is a way to thin the herd a little bit and prevent the longer-term consolidation of foreign fighters in places for long periods of time," he said. "That's the effort that we're undergoing right now. It is a process that we're working on."

The United States isn't upset that Britain has yet to agree to take the two militants from the Beatles cell, said one US official, who acknowledged that the United States faces legal and logistical complexities in another case, this one involving a US citizen.

The American, whom the government has not identified, was captured in Syria last year and is being held by the US military in Iraq.

The American Civil Liberties Union filed a lawsuit against Gen Mattis last week on behalf of the detainee in the US District Court for the District of Columbia.

In the filing, the ACLU said the US military lacks legal authority to detain the American citizen for months abroad without charge.

Similar legal challenges would likely arise in any effort to move such detainees to Guantanamo Bay.

Some US officials are concerned that a failure to deal with captured ISIS fighters properly could energise Sunni extremists in the longer term.

The officials want to avoid a repeat of the situation at Camp Bucca, a detention centre in Iraq that US forces took over in 2003. For years, Al-Qaeda operatives organised while in detention and eventually established ISIS.

The Pentagon has stressed that countries can't simply leave ISIS detainees on the battlefield.

"Doing nothing is not an option," Gen Mattis said.