By David Brunnstrom

BRUSSELS, June 7 (Reuters) - The U.S. government is looking at options to appeal a surprise decision by military judges this week to dismiss war crimes charges against two prisoners held at Guantanamo Bay, a U.S. government lawyer said on Thursday.

The U.S. military judges dropped all war crimes charges on Monday against the only two Guantanamo prisoners facing trial -- Canadian Omar Khadr and Salim Ahmed Hamdan of Yemen, an accused driver and guard for al Qaeda leader Osama bin Laden.

The judges said they lacked jurisdiction because the defendants were classified as "enemy combatants" rather than "unlawful enemy combatants", as required by the system set in place by Congress.

They gave the Bush administration 72 hours to decide on an appeal.

"Right now the government is looking at a number of different options," John Bellinger, legal adviser to the U.S. State Department, told a news briefing in Brussels.

He said the options included asking the judges who made the rulings to reconsider them based on additional information provided by the government, or asking the Combatant Status Review Tribunals to consider whether the men were unlawful enemy combatants -- something they had not previously been asked to determine.

Advocates for the detainees hailed Monday's decision as a watershed that would unleash legal challenges to the 2006 U.S. law that established the current military tribunal system.

Bellinger rejected this.

"I don't think we're back to square one," he said.

"I think the decision on Monday by the two judges reflect a technical legal obstacle that will require further work. It does not, as some critics are suggesting, bring the whole system into question ... "

"There was a mismatch between the rules of the Combatant Status Review Tribunals and the jurisdictional requirements of the Military Commissions Act. So it's a technical legal mismatch that will be an additional hurdle for us to go through."

The rulings do not affect U.S. authority to indefinitely hold about 380 foreign suspects at the Guantanamo Bay naval base in southeast Cuba, a source of criticism over how the United States is prosecuting its war on terrorism.

But advocates and lawmakers in Congress said it was time to restore the rights of the prisoners to challenge their detention in U.S. courts, which Congress revoked last year.

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