The secretive US military prison once housed hundreds of suspected militants captured by US forces during America's so-called "war on terror" following the 9/11 attacks by Al-Qaeda in 2001.

Businessman Saif Ullah Paracha was detained in 2003 in Thailand and accused of financing the jihadist group, but he has maintained his innocence and claimed a love for the United States.

Like most detainees at Guantanamo, Paracha -- aged 75 or 76 -- was never formally charged and had little legal power to challenge his detention.

"The Foreign Ministry completed an extensive inter-agency process to facilitate repatriation of Mr. Paracha," Pakistan's foreign office said in a statement on Saturday.

"We are glad that a Pakistani citizen detained abroad is finally reunited with his family."

Paracha's arrival comes after US President Joe Biden last year approved his release, along with that of another Pakistani national Abdul Rabbani, 55, and Yemen native Uthman Abdul al-Rahim Uthman, 41.

The statement from the Pakistani foreign ministry did not mention Rabbani.

A statement from the US Department of Defense said Paracha was released after it was determined his detention "was no longer necessary to protect against a continuing significant threat to the security of the United States".

The Pentagon thanked Pakistan for its "willingness... to support ongoing US efforts focused on responsibly reducing the detainee population and ultimately closing the Guantanamo Bay facility".

Biden is under pressure to clear out uncharged prisoners at Guantanamo and move ahead with the trials of those accused of having direct ties to Al-Qaeda.

The Pentagon statement said 35 detainees now remain at the facility, with 20 eligible for transfer and nine still facing charges in a military tribunal.

Among the remaining detainees on trial are several men who allegedly had direct roles in 9/11 and other Al-Qaeda attacks.

Paracha, who studied in the United States, had an import-export business supplying major US retailers.

US authorities accused him of having contact with Al-Qaeda figures, including Osama bin Laden and Khalid Sheikh Mohammad.

In 2008, Paracha's lawyer said the businessman had met bin Laden in 1999, and again a year later, in connection with the production of a television programme.

Reprieve, a UK-based human rights charity, described Paracha as a "forever prisoner".