AFP reports citing their lawyers and prison officials.
"My client and other men have reported that most of the detainees in Camp 6 are on strike, except for a small few who are elderly or sick," said Pardiss Kebriaei, a New York lawyer representing Ghaleb Al-Bihani, a Yemeni detainee.
Camp 6 houses the majority of the 166 detainees still incarcerated at Guantanamo -- estimated at about 130 men -- who usually don't pose any disciplinary problems or are regarded as a particular risk.
High-profile detainees such as Khalid Sheikh Mohamed, the mastermind of the September 11, 2001 attacks on New York and Washington, are held in a different part of the camp.
Interviewed by AFP, Robert Durand, director of public affairs for the Joint Task Force Guantanamo, said nine detainees were engaged in hunger strikes, five of whom were being fed through tubes inserted into their stomachs.
However, he indicated that "refusing delivered food does not make a detainee a hunger striker, not eating does.
"Detainees or an entire cell block may refuse to take any of the fresh, hot meals delivered, but we observe them eating from the ample amounts of food they have in the cell block," he added.
According to Kebriaei, her client -- on hunger strike for 30 days -- has "lost over 20 pounds and has been told by medical personnel that his health is in serious danger as he is also a diabetic."
Another lawyer, Barry Wingard, said one of his three clients, Kuwaiti Fayez Al-Kandari, lost 12 kilos (26 pounds) in three and a half weeks. All three were on hunger strike, he added.
Twelve lawyers -- including Kebriaei of the Center for Constitutional Rights -- have sent a letter to the commander of Guantanamo, Rear Admiral John Smith, to denounce "a matter that appears to be rapidly deteriorating and reaching a potentially critical level."
"We have received reports of men coughing blood, being hospitalized, losing consciousness, becoming weak and fatigued," said the letter, a copy of which was obtained by AFP.
"We understand that Arabic interpreters employed by the prison have been searching the men's Korans in ways that constitute desecration according to their religious beliefs, and that guards have been disrespectful during prayer times."
However, Durand pointedly rejected suggestions that copies of the Koran had been mistreated.
"To be clear: there have been no incidents of desecration of the Koran by guards or translators, and nothing unusual happened during a routine search for contraband," Durand said.
"No JTF-Guantanamo guard touches any detainee's Koran at any time. The Koran is treated with the utmost respect.
"We take allegations of Koran abuse seriously, and we also watch for manufactured claim of Koran abuse by detainees or outsiders."
Durand also said the number of hunger strikers was not exceptional and had been higher in the past.
The hunger strike comes after a disturbance in the camp in January, which led to a rubber bullet being fired at Guantanamo for the first time since 2006. The only person hit by the bullet escaped injury.
The detention facility at Guantanamo was opened in 2002 to house prisoners rounded up in the "War on Terror" of President George W. Bush's administration following the 9/11 attacks.