The plea deal with Majid Shoukat Khan, 31, would mark the first with a "high-value" detainee who had been detained by the CIA at a secret prison abroad before being transferred to the US-run detention center in Cuba.
Khan, who had been a legal resident in the United States, was charged with conspiring with al Qaeda to attack the United States and Indonesia as well as plotting to assassinate former Pakistani president Pervez Musharraf. He has been held at Guantanamo Bay since 2006.
Under the deal with military prosecutors, Khan -- who had previously faced a possible life sentence if convicted -- could eventually be released from the Guantanamo prison, the Post reported, quoting unnamed officials.
A Pentagon spokesman would not confirm the plea agreement, only saying that an arraignment hearing was set for next week on February 29.
"Mr. Khan has the right to enter into any legal arrangement he chooses," said Lieutenant Colonel Todd Breasseale.
Khan's lawyer, Jonathan Dixon, told AFP: "I cannot confirm or deny it, I have no comment on this case."
Khan agreed to testify at military commission trials over the next four years, and could then be transferred to Pakistan after that, according to the Post.
It was possible Khan could be asked to testify in the trials of Abd al-Rahim al-Nashiri, the alleged Saudi mastermind of the 2000 attack on the USS Cole, or in the cases of five other detainees charged with planning the September 11, 2001 attacks on New York and Washington.
Trials have yet to be scheduled for the confessed mastermind of the 9/11 attacks, Khalid Sheikh Mohammed, and four alleged co-conspirators.
Nashiri's case is expected to start later this year. He is accused of planning and preparing the October 2000 attack on the US Cole in Yemen's port of Aden, in which militants riding an explosives-laden skiff blew a massive hole in the naval destroyer, killing 17 sailors and wounding 40 more.
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