(FILES) This image grab taken from a propaganda video released on July 5, 2014 by al-Furqan Media allegedly shows the leader of the Islamic State (IS) jihadist group, Abu Bakr al-Baghdadi, aka Caliph Ibrahim, adressing Muslim worshippers at a mosque in the militant-held northern Iraqi city of Mosul. 
The Russian army on June 16, 2017 said it hit Islamic State leaders in an airstrike in Syria last month and was seeking to verify whether IS chief Abu Bakr al-Baghdadi had been killed. In a statement, the army said Sukhoi warplanes carried out a 10-minute night-time strike on May 28 at a location near Raqa, where IS leaders had gathered to plan a pullout by militants from the group's stronghold.
 / AFP PHOTO / AL-FURQAN MEDIA / --/AFP/Getty Images
ISIS leader seemingly breaks silence
02:38 - Source: CNN
CNN  — 

The American Civil Liberties Union is calling on the Trump administration to protect the rights of a US citizen detained for allegedly fighting for ISIS in Syria.

In a letter Friday to Secretary of Defense James Mattis and Attorney General Jeff Sessions, the civil rights group urged “immediate action to ensure that the United States’ conduct with respect to the detainee complies with the Constitution, and domestic and international law.”

“If the government has legitimate grounds to suspect the citizen fought with ISIS, he should immediately be transferred to the federal criminal justice system for criminal charges,” ACLU Executive Director Anthony Romero wrote. “On no account should the Defense Department resurrect the past policy of “enemy combatant” detention of US citizens, which proved to be a legal and moral failure.”

On Thursday, a Pentagon spokesman said the International Committee of the Red Cross had been notified of the detention of the US citizen, who is alleged to have been fighting with ISIS in Syria.

“In accordance with applicable Policy Directives and guidance, the International Committee of the Red Cross was notified of the detention earlier this week and is expected to meet with the detained unlawful enemy combatant in the near future,” Air Force Major Ben Sakrisson told CNN.

The humanitarian group is “following up with the US authorities to organize a visit, and anticipate being given timely access to visit the individual in accordance with US law and policy,” Marc Kilstein, a spokesperson for the ICRC in Washington, told CNN. Kilstein said the ICRC will meet the detainee in private and said “per our confidential working method, we will discuss any concerns regarding US detention privately and bilaterally with the US Government.”

Earlier this month, Marine Corps Major Adrian Rankine-Galloway told CNN that Syrian Democratic Forces, the US-backed Kurdish-led group fighting ISIS in that country, turned over to US forces an American citizen who surrendered around September 12.

“The US citizen is being legally detained by Department of Defense personnel as a known enemy combatant,” he added. The department has not said where the American is being detained.

The ACLU called for the accused American to be tried in the US criminal justice system.

“If the US citizen is indeed alleged to have fought with ISIS, the only lawful and legitimate option is to transfer him immediately to the federal criminal justice system for prosecution,” Romero wrote in his letter.

Romero also wrote that the path that the government follows “with respect to the US citizen in Defense Department custody is a critical test for this administration’s adherence to the rule of law.”

When asked for comment on the letter, Pentagon spokesman Sakrisson said the Department of Defense “will respond appropriately once the letter is officially received.”

“The disposition of the detained unlawful enemy combatant will be deliberated with the appropriate agencies; in the interim, the individual remains in DoD custody,” he added, referring CNN to the Department of Justice “regarding the appropriateness of pursuing criminal charges.”

Justice Department spokesperson Wyn Hornbuckle told CNN: “As a part of the President’s national security team, the Department of Justice recognizes the wide range of tools and authorities that the President possesses to protect our national security and to defeat our terrorist adversaries. All options remain on the table, and the Justice Department will continue to use every lawful investigative and prosecutorial tool to achieve these objectives.”

This isn’t the first US citizen to be detained by US allies in the region for allegedly fighting for ISIS. In March 2016, a US citizen was captured in Iraq by US-backed Kurdish Peshmerga forces.

The number of US citizens traveling to Iraq and Syria to join ISIS is thought to be much smaller than those from other countries and regions, such as Western Europe. Coalition officials have said far fewer foreign fighters have joined ISIS in recent months as the terror group suffers setbacks on the battlefield.

CNN’s Laura Jarrett contributed to this report.