Some of the 89 striking detainees at Guantanamo have not eaten for a month, said Guantanamo detention mission spokesman Sgt. Justin Behrens. The others have refused at least nine consecutive meals, he said.
Fifteen have been hospitalized and 13 of those were being fed through tubes, Behrens said in a written response to questions from The Associated Press. Medics are monitoring all 89 and checking their vital signs daily, he added.
Previously, the military has said that 76 inmates were participating in the hunger strike.
British lawyer Clive Stafford-Smith, who represents one of the hunger strikers, Briton Omar Deghayes, 36, warned Friday that some of the inmates were willing to starve themselves to death.
"People are desperate. They have been there three years. They were promised that the Geneva Conventions would be respected and various changes would happen and, unfortunately, the (U.S.) government reneged on that," Stafford-Smith said.
"Sadly, it is very hard to see how a very obstinate military and a very desperate group of prisoners are ever going to come to an agreement."
Guantanamo prison spokesman Maj. Jeff Weir said the military would not allow the detainees' conditions to become life-threatening.
"Basically, if you stop eating and wait several weeks or months, it is a slow form of suicide," Weir told British Broadcasting Corp. radio and television. "No detention facility in the world will deliberately let their people commit suicide, so we can't let that happen."
Weir said he did not know the reason behind the hunger strike.
"As far as their reasons for hunger striking it seems to be a myriad of different reasons that they all have, the largest one seems to be like they want to protest their continued (detention)," he said. "Their future is uncertain from a legal point view so they are trying to find out exactly what their future entails."
The prison at Guantanamo opened in January 2002 and now holds around 520 prisoners from 40 countries; more than 230 others have been released or transferred to the custody of their home governments. Many were captured during the U.S. war in Afghanistan after the Sept. 11 attacks.