WASHINGTON, Dec 18 (Reuters) - The Pentagon is working on a proposal to shut the U.S. military prison at Guantanamo Bay, Cuba, that would be available to President-elect Barack Obama once he takes office, a defense official said on Thursday.

U.S. Defense Secretary Robert Gates asked his staff to come up with a plan to address the thorny issue of how to close the prison in southeast Cuba that Obama has pledged to shutter after he officially enters the White House on Jan. 20.

Guantanamo has about 250 detainees including Khalid Sheikh Mohammed, accused of masterminding the Sept. 11 attacks. The prison has come to symbolize aggressive detention practices that opened the United States up to allegations of torture.

"(Gates) has asked his team for a proposal on how to shut it down -- what would be required specifically to close it and move the detainees from that facility while at the same time, of course, ensuring that we protect the American people from some dangerous characters," Pentagon press secretary Geoff Morrell told reporters.

"If this is one of the president-elect's first orders of business, the secretary wants to be prepared to help him as soon as possible," he said.

Like Obama, Gates has said Guantanamo should be closed. Appointed by President George W. Bush, Gates has agreed to stay on in the Obama administration. He has said closing the prison will require legislation by the U.S. Congress.

"The request has been made, his team is working on it so that he can be prepared to assist the president-elect should he wish to address this very early in his tenure," the press secretary said.

Among the issues to be settled before the prison closes are what to do about the military commissions system, trials under way and about 60 detainees that U.S. officials have approved for transfer to their home countries.

The Guantanamo tribunals are scheduled to reconvene on Jan. 19 for pretrial hearings for Canadian captive Omar Khadr, who is set for trial the following week on charges of murdering a U.S. soldier with a grenade during a firefight in Afghanistan. (Reporting by David Morgan with additional reporting by Jane Sutton in Miami, Editing by Howard Goller)

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