The detainees, among some 500 al-Qaida and Taliban suspects held at the US Navy base, have refused at least nine consecutive meals, the Joint Task Force at Guantanamo said in a statement on Thursday.
Attorneys for some detainees said Guantanamo prisoners had planned in late June to begin a hunger strike to express frustration over "their indefinite detention and the inhuman conditions at Guantanamo", according to a statement from the New York-based Centre for Constitutional Rights (CCR).
Human rights groups have repeatedly criticised the United States for indefinitely detaining suspects at the Guantanamo prison camp, which was opened on the base in January 2002.
Former prisoners have said they were tortured there. Many of the prisoners at Guantanamo, accused of having ties to the al-Qaida group or the ousted Taliban in Afghanistan, have been held for more than three years. Only a handful have been charged.
CCR, citing recently declassified notes taken by attorneys who visited inmates, said the hunger strike is a "peaceful, nonviolent strike until demands are met" and calls for "starvation until death".
"[The US] government continues to deprive them of the basic dignities that every human being is entitled to"
Centre for Constitutional Rights
US military officials said detainees who refuse food are given medical treatment including intravenous hydration, water, the sports energy drink Gatorade, a nutritional supplement called Ensure and are admitted to hospital if needed.
"Indications are that this is a temporary effort by some detainees to protest their continued detention," they said in the statement.
CCR, which helps represent Guantanamo inmates, said the hunger strikers also planned to boycott showers and recreational time and some would refuse to wear clothes.
The prisoners are demanding clean food and water, better medical care, more access to sunlight, contact with relatives, greater respect for their religion - including an end to desecration of the Koran - and fair trials with proper legal representation, the CCR statement said.
"They have languished in legal limbo for years with no fair trial and no definitive resolution of their legal status," CCR deputy legal director Barbara Olshansky said. "All the while, our government continues to deprive them of the basic dignities that every human being is entitled to."
The US government has released more than 200 detainees from Guantanamo, some of whom were transferred to detention centres in their home countries.