A prisoner attempted suicide for the third time at the U.S. military's detention center at Guantanamo Bay (search), Cuba, bringing the total number of attempts to 28, an official said Tuesday.

Guards prevented serious injury when the man made the attempt in a recently opened mental health wing Monday, said Maj. Paul Caruso, a spokesman for the detention mission.

It was the first attempt reported in the mental ward, which opened in March and now holds about two dozen detainees under monitoring by a psychiatrist, psychologist, four psychiatric nurses and others.

Caruso declined to provide details on how the man tried to commit suicide or whether he was one of two prisoners who attempted suicide on May 17 and May 19.

Eighteen detainees have attempted suicide at the U.S. outpost in eastern Cuba, some of them repeatedly and most by trying to hang themselves with sheets or pieces of clothing, officials say.

Some 680 detainees from 42 countries are being held on suspicion of links to the Al Qaeda (search) terrorist network and the fallen Afghan Taliban regime. None have been formally charged and they are not allowed lawyers.

Human rights groups have decried the prisoners' indefinite detention and raised concerns about whether conditions or interrogations are driving some to attempt suicide.

Eighteen of the 28 suicide attempts at Guantanamo have come this year.

Military officials said the mental ward was opened in an effort to treat detainees who were mentally ill before their capture.

Under rare circumstances, if a detainee in the mental ward is deemed a threat to himself or others, he can be bound with restraints on his arms and legs, officials say.

The U.S. military is preparing for possible trials of some detainees before tribunals and has appointed a chief defense lawyer and chief prosecutor. It's unclear how soon the tribunals could begin.