SAN JUAN, PUERTO RICO - The number of Guantánamo Bay detainees participating in a hunger strike has ballooned from three to around 75, the U.S. military said Monday, revealing growing defiance among prisoners held for up to 4 -1/2 years with no end in sight.

Navy Cmdr. Robert Durand called the hunger strike at the U.S. naval base in southeastern Cuba an "attention-getting" tactic to step up pressure for the inmates' release and said it might be related to a May 18 clash between detainees and officers that injured six prisoners.

"The hunger-strike technique is consistent with al-Qaida practice and reflects detainee attempts to elicit media attention to bring international pressure on the United States to release them back to the battlefield," Durand said from Guantánamo Bay.

The United States now holds about 460 people at Guantánamo on suspicion of links to al-Qaida or the Taliban. But human-rights groups say innocent people have been sent to the jail. Defense lawyers said the hunger strike, which began last year, reflects increasing frustration among men who have little contact with the world outside the remote prison.

"I think it is escalating because the people down there are getting more and more desperate," said Bill Goodman, legal director for the New York-based Center for Constitutional Rights, which represents many of the detainees. "Obviously, things have reached a crisis point."

The military did not release the names of the striking detainees, and lawyers said they have no way of learning if their clients are involved until they can visit the base.

"All these men want is a chance to have a trial," said Zachary Katznelson, an attorney for Reprieve, a British human-rights group that represents 36 Guantánamo detainees. "If they are guilty, punish them. If not, then send them home."

Only 10 of the prisoners have been charged and face trial before military tribunals. The U.S. Supreme Court is expected to rule in June whether President Bush overstepped his authority in ordering the tribunals.

Four of the detainees on hunger strike are being force-fed, including the three who were participating in the protest before the recent increase, Durand said.