The prison came to symbolise the excesses of the US's "war on terror" because of harsh interrogation methods that critics have said amounted to torture.
Inmates who have been held for years in the US's Guantanamo Bay detention facility in Cuba are showing signs of "accelerated ageing", a senior official of the International Committee of the Red Cross has said.
"We're calling on the US administration and Congress to work together to find adequate and sustainable solutions to address these issues," Patrick Hamilton, the ICRC's head of delegation for the United States and Canada, said on Friday.
"Action should be taken as a matter of priority."
Hamilton's comments came after a visit to the facility in March following a 20-year hiatus.
He said he was "struck by how those who are still detained today are experiencing the symptoms of accelerated ageing, worsened by the cumulative effects of their experiences and years spent in detention".
He called for detainees to receive adequate mental and physical health care and more frequent family contact.
READ MORE How US detained Ghassan al Sharbi for 20 years on false terror charges
The US State Department did not immediately respond to a request for comment.
The Guantanamo camp was established by Republican President George W. Bush in 2002 to house foreign terrorism suspects following the 2001 hijacked plane attacks on New York and the Pentagon that killed about 3,000 people.
It came to symbolise the excesses of the US "war on terror" because of harsh interrogation methods that critics have said amounted to torture. There were 40 detainees when President Joe Biden, a Democrat, took office in 2021. The Biden administration has said it wants to close the facility but has not presented a plan for doing so.
The repatriation of two brothers to Pakistan occurred in February, and 30 prisoners remain. Hamilton also urged Washington to resolve the fate of the detainees, urging action to transfer those out who were eligible.