Democratic lawmakers called on US President Joe Biden Thursday to immediately shut down the Guantanamo prison and either release or place on trial in federal courts the 39 remaining "war on terror" detainees there.

With the United States approaching the 20th anniversary of the September 11 attacks, 75 lawmakers signed a letter saying the US Navy-run prison in Guantanamo Bay, Cuba was in disrepair, costly and a two-decade-old human rights embarrassment to the country.

The prison has become a symbol of the excesses in the "war on terror" launched after the 2001 attacks.

"The prison at Guantanamo has held nearly 800 prisoners throughout its history but currently holds only 39 men, many aging and increasingly infirm," they wrote.

"According to reports, the prison costs over $500 million per year to operate, at a staggering annual cost of $13 million per prisoner," they said.

"The continued operation of the prison is a stain on our international reputation and undermines our ability to advocate for human rights and the rule of law," they said.

The prison was activated in late 2001 when the United States combed the world for members and supporters of Al-Qaeda complicit in the September 11 and other attacks against US assets and facilities.

But detaining and secretly rendering hundreds of men to Guantanamo, subjecting many to torture, and holding them for years without charges or due process heavily tainted Washington's demands for justice after the 9/11 attacks.

Only a handful of prisoners have been charged under the military courts system set up for Guantanamo, and their cases have mostly ground to a halt.

The letter cited Biden's support for closing Guantanamo when he became vice president in 2009, which was reiterated after he became president in January.

"After nearly two decades, and numerous efforts at reform, the military commission process remains dysfunctional," the letter said.

When Biden became president, 40 detainees remained at the prison. One was released to his home in Morocco in July, and another 10 have been approved for release -- some for more than four years -- and away arrangements for their repatriation or transfer to a third country.

Twelve, including Khalid Sheikh Mohammed, called the architect of the 9/11 attacks, are in the slow-moving military tribunal system. Just two have been convicted over nearly two decades.

The other 19 are in limbo: they haven't been charged or granted release.

After stalling under Biden's predecessor Donald Trump, the military tribunals began holding hearings again last month.

In September, the case of Khalid Sheikh Mohammed is scheduled to resume with a pre-trial hearing.