More than two dozen men arrived at the U.S. prison at Guantanamo Bay in Cuba on Friday, pushing the number of terror suspects at the naval base to about 650.

The arrivals came a day after the Pentagon reported a recent rise in suicide attempts among detainees at the base, drawing criticism from a human rights group that says U.S. interrogations may be to blame.

About 25 men arrived at Guantanamo about a military cargo plane, said Army Maj. Paul Caruso, a spokesman for the detention mission. Officials refused to give an exact number or say where the plane came from.

"They were processed and taken to Camp Delta," the U.S. detention compound at Guantanamo, Caruso said.

Before Friday's arrivals, U.S. officials estimated the prisoner count at about 625.

There have been 15 suicide attempts among Guantanamo detainees since the United States began holding them about a year ago. At least five detainees have tried to hang themselves in the past three weeks, the Pentagon said Thursday.

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

The most serious attempt was made Jan. 16 by a man in his 20s. Caruso said the man was in stable condition Friday and was being closely monitored in the hospital. Authorities have notified his home country.

The United States is holding the detainees on suspicion of links to the fallen Taliban regime of Afghanistan or Al Qaeda terror network.

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