Suicide attempts among terror suspects held at Guantanamo Bay in Cuba are on the rise, the Pentagon said Thursday. A human rights group has called for an investigation.

At least five detainees at the naval base have tried to hang themselves in the past three weeks, Pentagon spokeswoman Lt. Cmdr. Barbara Burfeind said Thursday in Washington.

The most serious attempt was made Jan. 16 by a man in his 20s who is now in stable but serious condition, Burfeind said. Authorities have notified his home country, but it is unknown whether his family has been contacted.

Human rights group Amnesty International urged U.S. authorities to investigate the suicide attempts to determine whether the manner in which interrogations are conducted may have contributed.

"I think when multiple prisoners attempt to take their own lives, clearly there is an indication of the uncertainty that surrounds their incarceration," said Alistair Hodgett, a spokesman for the group in Washington.

Amnesty International says it has received reports the detainees are interrogated at all hours and sometimes forced to kneel. Military officials have refused to talk about the interrogations except to say that valuable information is still being gleaned.

The United States is holding about 625 detainees from 41 countries who it suspects are linked to the fallen Taliban regime in Afghanistan or the Al Qaeda terror network.

The detainees, treated by the U.S. government as enemy combatants, are held indefinitely without being formally charged or tried, and they are interrogated without access to lawyers.

There have been 15 suicide attempts, including one detainee who has tried to kill himself twice, since the detention mission began about a year ago.

Treating the detainees, many of whom have seen battle, poses a challenge for U.S. staff dependent on translators and unfamiliar with some cultures.

"Medical and psychiatric teams are working to try to prevent further injury or attempts," Burfeind said. She declined to give details.

Navy Capt. Al Shimkus, who heads the camp hospital at the base on Cuba's eastern tip, has been building a team of mental health nurses and medics, joined recently by a psychiatrist.

Several detainees suffer from mental illnesses including post-traumatic stress disorders, depression and schizophrenia. In May, one schizophrenic was sent home to Afghanistan.

Many have been given medication, although Guantanamo authorities refuse to discuss details. None agreed to interviews Thursday.