WASHINGTON (Reuters) - President George W. Bush asked Congress on Tuesday to consider giving him powers to use the military to enforce quarantines in case of an avian influenza epidemic.

He said the military, and perhaps the National Guard, might be needed to take such a role if the feared H5N1 bird flu virus changes enough to cause widespread human infection.

"If we had an outbreak somewhere in the United States, do we not then quarantine that part of the country? And how do you, then, enforce a quarantine?" Bush asked at a news conference.

"It's one thing to shut down airplanes. It's another thing to prevent people from coming in to get exposed to the avian flu. And who best to be able to effect a quarantine?" Bush added.

"One option is the use of a military that's able to plan and move. So that's why I put it on the table. I think it's an important debate for Congress to have."

Bird flu has killed more than 60 people in four Asian nations since late 2003 and has been found in birds in Russia and Europe.

Experts fear that the H5N1 bird flu virus, which appears to be highly fatal when it infects people, will develop the ability to pass easily from person to person and would cause a pandemic that would kill millions.

"And I think the president ought to have all ... assets on the table to be able to deal with something this significant," Bush said.

He noted that some governors may object to the federal government commandeering the National Guard, which is under state command in most circumstances. 

POLICE DUTIES BANNED

"But Congress needs to take a look at circumstances that may need to vest the capacity of the president to move beyond that debate. And one such catastrophe or one such challenge could be an avian flu outbreak," Bush said.

The active duty military is currently forbidden from undertaking law enforcement duties by the federal Posse Comitatus Act.

That law, passed in 1878 after the U.S. Civil War, does not prohibit National Guard troops under state control from doing police work. But, unless the law is changed, it would keep them from doing so if they were activated by Washington under federal control.

While the law allows the president to order the military to take control and do police work in an extreme emergency, the White House has been traditionally reluctant to usurp state powers.

Pentagon spokesman Bryan Whitman told reporters he was not aware of any current planning by the military to help respond to a flu pandemic.

But he noted that after Hurricanes Katrina and Rita devastated the Gulf region, Bush had asked Congress to consider giving the military control over initial response in dealing with major natural or other domestic disasters.

"Obviously the (Defense) Department has a tremendous amount of capability in a lot of areas. And we are a large force," Whitman said, noting also that the military had deployed field hospitals to Louisiana after the hurricanes.

Health experts are working to develop vaccines that would protect against the H5N1 strain of flu, because current influenza vaccines will not.

And countries are also developing stockpiles of drugs that can reduce the risk of serious disease or even sometimes prevent infection -- but supplies and manufacturing capacity are both limited.

Bush said he was involved in planning for an influenza pandemic, which experts say will definitely come, although they cannot predict when, or whether it will be H5N1 or some other virus.

(Additional reporting by Maggie Fox)