WASHINGTON, Feb. 12 - Senior Defense Department officials said Thursday that they were planning to keep a large portion of the detainees at Guantánamo Bay, Cuba, there for many years, perhaps indefinitely.

The officials said they would soon set up a panel to review those long-term prisoners' cases annually to determine whether the men remained a threat to the United States or could be released. The officials described the panel as a "quasi-parole board" that would comprise three members before whom prisoners could personally plead their case for release. At the same time, the officials said, in the coming months they will continue to release to the home governments many other prisoners deemed not to be a continuing danger.

The officials spoke as part of a Pentagon effort to counter sharp criticism by members of human rights groups and foreign governments about the situation at Guantánamo, where some 650 people, most of them captured in Afghanistan, are being held under maximum security, some as long as two years without being charged with any offense. Defense Secretary Donald H. Rumsfeld is scheduled to discuss the matter in a speech Friday in Miami.

One senior Defense Department official who spoke on condition of anonymity said critics in the United States and abroad had greatly misunderstood the situation at Guantánamo and the need to detain so many people without charging them.

"We feel very much like we are in an active war," said the official, asserting that the civilian law enforcement model in which people are prosecuted for crimes or set free did not apply. "What we're doing at Guantánamo is more understandable in the war context," the official said.

The official said that while some critics worried about the rights of the detainees, the Pentagon was more concerned with "the rights of the soldiers having these people not going back to the battlefield" and the rights of the soldiers' families not to have their relatives face the same men in combat.

Many of the prisoners, a senior military official said, remain committed to indiscriminately killing American civilians and soldiers and would be too dangerous to release.

The argument that the detentions should be seen in a wartime context is, however, unlikely to satisfy many critics. Michael Ratner, the president of the Center for Constitutional Rights, said that "the idea that you could theoretically keep someone locked up forever under these circumstances is reprehensible." Mr. Ratner, whose New York-based organization has challenged the Guantánamo detentions, said he was taken aback by what he called the administration's brazenness.

"It's nothing to do with law as any person should understand it, at least since the Magna Carta," he said. "How do you know without a trial that these people are even dangerous? It all depends on the military's word."

But the United States officials insisted many prisoners at Guantánamo were "the worst of the worst." They said that over the course of many months of interrogation and intelligence work, they had come to believe that many of the people being held were senior operatives of Al Qaeda who had been involved in active plots against Americans.

Although they are impossible to verify independently, one defense official described several individual cases including one man described as a bodyguard for Osama bin Laden, another who had been involved in planning attacks on tankers in the Persian Gulf, another who officials said was involved in attacks against United States embassies in Africa in 1998 and two others who were involved in financing Qaeda operations.

The intelligence effort was painstaking, defense officials said, noting that one prisoner had 13 aliases.

Those believed to be involved with Al Qaeda, the officials said, could be brought before military tribunals for which elaborate rules have been established but which have not yet been put into effect.

"But whether a person is to be charged before a military commission is not the reason we're holding them," said the senior defense official. The official said it was possible that an individual could be convicted by a tribunal and serve a five-year sentence and then not be released if he were judged to remain a danger.

The panel that would evaluate long-term prisoners' fitness to be released could hear not only from the detainee, but also from the prisoner's home government, the officials said. The panel's decisions would eventually be reviewed by the secretary of defense.

The officials said there was no decision yet on who would serve on the panel or whether they would be in the military or civilians.

One administration official described the plan for dealing with Guantánamo as having increasingly large transfers of less dangerous prisoners to their home governments for possible prosecution, thus winnowing down the population to a hard-core group.

The senior defense official who spoke Thursday said the category of those who could eventually be repatriated could include anywhere from 100 to 300 prisoners.

The military has released more than 80 Guantánamo prisoners to their home governments so far, saying they were deemed not to be a threat or of further intelligence use. The administration official said that scores more would be transferred in the next few months.

The administration has been in intense negotiations with several governments who might agree to accept the return of their citizens.

Military authorities are building a hard-walled traditional prison alongside the corrugated metal units that have housed detainees for nearly two years at the Guantánamo naval base on the southeastern tip of Cuba. The prison, which is expected to be ready this summer, will be able to house about 100 people, military officials have said.

Although the rules for military tribunals include the possibility of capital punishment, officials said there were no plans to build an execution chamber in the new prison.

The government has argued that the detainees are not entitled to American constitutional protections because the Guantánamo base is outside United States territory. Two courts have supported that view and the Supreme Court is scheduled to hear arguments in April.