They have refused at least nine meals over the past three days, officials said.
The detainees refusing meals have been given intravenous hydration, Gatorade, water and the nutritional supplement Ensure, the military said. Some might be admitted to a hospital.
The Pentagon's version of the strike contrasted somewhat with the accounts of two Afghans released from the prison this week. On Wednesday, they alleged that more than 180 Afghans were on a hunger strike to protest mistreatment at the U.S. detention facility.
Habir Russol and Moheb Ullah Borekzai, who said they left the prison Monday and were flown to Afghanistan before being freed, said they did not participate in the hunger strike. They did not say how they knew others were refusing to eat.
Russol said 180 Afghan prisoners "are not eating or drinking." He and Borekzai said the men were in the 14th or 15th day of their fast.
Borekzai later said the detainees were protesting because "some of these people say they were mistreated during interrogation. Some say they are innocent."
Neil Koslowe, a Washington-based lawyer for 12 detainees from Kuwait, said several inmates told him during a June 20-24 visit to Guantanamo that there was a widespread hunger strike over the amount and quality of their drinking water.
The two Afghans released this week said they had been accused of being members of the former Taliban regime, but both said they were not. Neither said how long they were detained.
The Pentagon announced Wednesday that seven Guantanamo detainees had been released and an eighth was placed in the custody of a foreign government. In addition to the two released Afghans, three Saudis, a Jordanian and a Sudanese were freed, it said.