The number of detainees on hunger strike at the US detention centre at Guantanamo Bay, Cuba, has grown and now involves 75 inmates, the US says.
Hunger strikes have flared periodically since 2002
Navy Cmdr Robert Durand said the new hunger strike was aimed at attracting media attention and may also be connected to a disturbance on 18 May.
Detainees started an on-off hunger strike last August to protest at their continued detention and conditions.
Rights groups have voiced concerns that the US has force-fed the strikers.
About 460 prisoners remain at Guantanamo, many of them captured in Afghanistan. Some have been held for nearly four years without charge.
The US military defines a hunger strike as missing nine consecutive meals and most of the 75 passed that mark on Sunday.
Most are refusing food but are drinking liquids.
Cmdr Durand said the hunger strike was not a new tactic at the detention centre and that most returned to full normal diets after media attention had passed.
He said the current protest may be designed to coincide with a series of hearings scheduled in June.
"This new hunger strike is likely a co-ordinated, but short-term, effort designed to coincide with the military commission hearings scheduled for the next several weeks as defence attorneys and media normally travel to Guantanamo to observe this process," Cmdr Durand said in a statement.
He said the gesture may also be related to an incident earlier this month when two detainees tried to commit suicide and several others clashed with guards.
Seventy-six detainees began a hunger strike in August. Since then the number has at times grown and then dwindled to a handful.
Three men who have been protesting since August and one of the recent group are being enterally fed, that is via a tube through the nose and into the stomach, the military says.
Defence lawyers have said many detainees stopped their protest because the US military adopted more aggressive measures to force feed them.
In March, more than 250 medical experts signed a letter condemning the US for force-feeding prisoners on hunger strike.
Earlier this month, the UN Committee against Torture called on the US to close Guantanamo and any other secret "war on terror" detention facilities abroad.
The Bush administration has denied allegations of abuse at Guantanamo, and the military says it provides safe, humane care and custody of the detainees.