Guantanamo Bay detainees are showing signs of "accelerated ageing", an official from the International Committee of the Red Cross has said.
"We call on the US administration and the Congress to work together to find appropriate and sustainable solutions to the question of Guantanamo," Patrick Hamilton, Research Director for the United States and Canada said after his first visit to Guantanamo in 20 years.
The ICRC chief said he was "particularly 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."
"More than 20 years later, their situation remains unresolved. Their physical and mental health needs are growing and becoming increasingly challenging."
Hamilton called on authorities to provide appropriate medical care for both mental and physical issues - which he said have been systematically neglected.
Guantanamo at its peak in 2003 held about 600 people whom the US considered terrorists.
Proponents of the detention facility's use claim it has prevented terror attacks, but critics say the military detention without trial and opaque court system subverted human rights and constitutional rights - and undermined the United States’ standing abroad.
Thirty detainees now remain at Guantanamo Bay, since authorities on Thursday released the last Algerian prisoner at the prison, Said Bakush.
Among those who remain, 18 are eligible for transfer if stable third-party countries can be found to take them, the Pentagon said.
Many are from Yemen, a country considered too plagued with war and extremist groups and too devoid of services for freed Yemeni detainees to be sent there.
Nine of the detainees are defendants in slow-moving military-run tribunals. Two others have been convicted.