The strike - which last weekend jumped from three participants to 75 - is now at its biggest point this year at the US prison on Cuba, where about 460 men are being held on suspicion of links to al-Qaida or the Taliban.
The US military on Thursday said the detainees were trying to pressure the United States to release them, but a human rights attorney described the strike as a desperate appeal for justice.
Six hunger strikers were being force-fed, Robert Durand, a Navy commander, said.
"All are being closely monitored by the ... medical staff and being counselled on the health effects of long-term hunger striking," Durand said in a statement from Guantanamo Bay.
Military officials said the hunger strikers are trying to gain public sympathy to pressure the United States to release them.
Call for help
Ben Wizner, an American Civil Liberties Union attorney who has been to Guantanamo Bay, said the growing hunger strike appears more like a call for help by detainees.
"The vast majority have never been charged with any crime, and have been prevented from communicating directly with the outside world," Wizner said in a telephone interview from New York. "So it may well be their attempt to ensure that the world is reminded of their unlawful detention."
A UN panel said on May 19 that holding detainees indefinitely at Guantanamo Bay violates the world's ban on torture. The panel said the United States should close the detention center.
Only 10 Guantanamo detainees have been charged with crimes.
The hunger strike comes amid increasing displays of defiance from the prisoners, who have been held for up to four and a half years, with many claiming their innocence.
On May 18, a detainee staged a suicide attempt to lure guards into a cellblock, where they were attacked by prisoners armed with makeshift weapons, the military said.