However, humanitarian organizations criticized the USA government's plan in releasing him in Syria, stating that this is equal to a death sentence and demanded transferring him to the U.S. given that he holds its nationality.
The American Civil Liberties Union said it would ask a federal judge to order the government not to release him in Syria, which the group called "a disgraceful way to treat an American citizen".
The Justice Department announced late Wednesday that US forces were preparing to release the man, a dual US-Saudi citizen identified only as "John Doe", into territory controlled by allied Syrian Democratic Forces as early as Saturday, after having held him as an alleged "enemy combatant" for nine months.
The unidentified man, referred to as John Doe in court filings, was captured in Syria in September by the Syrian Democratic Forces (SDF), a USA -backed alliance of Kurdish and Arab militia groups fighting Islamic State and Syrian government forces.
But the man "did not identify a preference between the two locations and would not agree to the release" as proposed to him, the filing said.
He was also listed as a "fighter" in an internal Islamic State document, the document said.
The man grew up in Saudi Arabia and also had Saudi citizenship, court filings show.
A spokesman for the Defense Department declined to comment, citing the pending litigation.
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By Friday it was clear that what started as a court case over whether the USA government had the legal authority to detain an American man suspected of fighting for ISIS had devolved into an argument over what obligations the US has to ensure his safety upon release.
At a hearing Friday, U.S. District Judge Tanya Chutkan asked both sides for more information before ruling and set a hearing for June 20.
The ACLU says the man, who once lived in Louisiana, was in Syria to chronicle the conflict and was trying to flee the violence when he gave himself up.
The Pentagon also disclosed that it had listened in on 18 telephone conversations between the man and his ACLU attorneys as part of "routine security monitoring of DOD telecommunications systems" - calling it an "inadvertent breach" of attorney-client confidentiality that it "regrets". But the ACLU argued that transferring him to Saudi Arabia, a country with poor due process protections, would prevent him from exercising his right to challenge the legality of his detention. "The court encouraged the government to find solutions that are not unsafe".
The US military has held "John Doe" since September.
Last month, in a victory for the detainee, a federal appeals court ruled that the USA could not "forcibly transfer" him to a third-party country such as Saudi Arabia, where he has dual citizenship, without proving to a court that he had fought for the enemy.
The United States previously said it had evidence that he had signed up as an Islamic State foreign fighter in 2014 and entered Syria in January 2015, according to court papers.
"What the government is offering our client is no release - it's a death warrant".