(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.

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