A spokesman for the controversial detention center said 89 prisoners were now on hunger strike, up from 75 reported on May 29.
Navy Commander Robert Durand said six were being force-fed through tubes.
Military officials have said the hunger strikers are trying to gain media attention and put pressure on the United States to release them. But critics have said the action is a sign of despair.
Some 460 men are being held at the prison on suspicion of links to Al-Qaeda or the Taliban.