WASHINGTON - The Obama administration said on Tuesday it will move some Guantanamo Bay detainees to an Illinois prison and hold U.S. military commission trials there in plans immediately criticized as risky by Republicans.

President Barack Obama's top national security aides said in a letter that the U.S. government will proceed with buying the Thomson Correctional Center in northwestern Illinois and bolstering security at it to house a limited number of detainees from Guantanamo as well as other federal inmates.

Illinois Democratic Senator Dick Durbin, who backed the move along with Illinois Governor Pat Quinn, put the number of Guantanamo detainees to be sent to Illinois at fewer than 100.

The administration plans to expand the security perimeter of the facility, make it the most secure in the country and hold U.S. military commission trials inside its walls.

The move is part of Obama's struggle to fulfill a campaign pledge to close the Guantanamo Bay, Cuba, U.S. naval detention camp prison, which was opened in 2002 after the September 11, 2001 attacks to house foreign terrorism suspects.

Not only will this help address the urgent overcrowding problem at our nation's federal prisons, but it will also help achieve our goal of closing the detention center at Guantanamo in a timely, secure, and lawful manner, said the letter to Quinn signed by Secretary of State Hillary Clinton and Defense Secretary Robert Gates, among others.

Obama has already acknowledged he will not be able to close the Guantanamo prison by the end of his first year in office in January because of the problem of where to put its remaining prisoners.

The United States holds 210 prisoners at Guantanamo and has released or transferred to other governments more than 555 others.

Republicans and others criticized Obama's plan as a security risk.

I have yet to hear one good reason why moving these terrorists from off our shores right into the heart of our country makes us safer, said Republican Senator Lamar Alexander.

House Republican leader John Boehner said the decision showed the Obama White House must've forgotten about those Americans killed on September 11, prompting White House spokesman Robert Gibbs to say the comment was far crazier than others he had seen from Obama's opponents.

CHANGE IN LAW NEEDED

Administration officials countered by saying U.S. prisons already house more than 340 inmates linked to international or domestic terrorism, and that al Qaeda has used the existence of Guantanamo Bay as a recruiting tool.

Congress enacted a law barring Guantanamo detainees from being brought onto U.S. soil except if they were going to be prosecuted. Democrats, who control both houses of Congress, are planning to lift that restriction if the administration comes up with an acceptable plan for dealing with the prisoners.

Senior administration officials who briefed reporters on the decision said current law allows bringing in Guantanamo inmates for putting them on trial, but a change in law would be required for transferring detainees to Thomson for any purpose other than prosecution.

The plans were announced less than a month after Obama generated controversy by agreeing to send alleged September 11 mastermind Khalid Sheikh Mohammed from Guantanamo Bay to New York to stand trial.

Republicans quickly signaled their opposition to the new move. Senate Republican leader Mitch McConnell said Americans and Congress had already rejected bringing terrorists to U.S. soil for long-term detention, and current law prohibits it.

House of Representatives Judiciary Committee Republican ranking member Lamar Smith of Texas said the move would give ...terrorist enemy combatants access to the same rights as U.S. citizens.

Once on U.S. soil, whether detained in a prison or awaiting trial, Gitmo terrorists can argue for additional rights under the Constitution that may make it harder for prosecutors to obtain a conviction, Smith said in a statement.

The administration's letter said that the president has no intention of releasing any detainees in the United States.

The Defense Department would operate a part of the prison, located in a rural area west of Chicago, devoted to housing the Guantanamo detainees.

The security of the facility and the surrounding region is our paramount concern, it said.

The facility was built in 2001 to maximum security specifications, and after acquisition it will be enhanced to exceed security standards at the country's only supermax prison in Florence, Colorado, where there has never been an escape or external attack, officials said.

Quinn and Durbin, briefed at the White House, said the prison has been sitting empty for eight years and the new plan would create 3,000 jobs.

This is an opportunity to dramatically reduce unemployment, create thousands of good-paying jobs and breathe new economic life into this part of downstate Illinois, they said.

(Additional reporting by Timothy Gardner and Ross Colvin; Editing by Eric Walsh)