* Program ensures U.S.-flagged ships available for military

* Celebrity captain among those asking Congress to act

* "We could lose our pants," says union official

By David Alexander

WASHINGTON, Oct 10 (Reuters) - An American merchant marine captain made famous by a Tom Hanks movie after being kidnapped by Somali pirates warned on Thursday that looming U.S. budget cuts could sink part of a program that keeps U.S.-flagged ships ready to ferry military supplies and aid around the world.

Richard Phillips, who was held hostage after Somali pirates seized his vessel in 2009, is backing a campaign to halt the budget cuts, saying they could reduce by a third the size of the 60-strong U.S. commercial fleet that regularly carries cargo for the military.

His ship, the Maersk Alabama was carrying U.S. food aid when it was boarded by the pirates.

Captain Steven Werse, an official at the International Organization of Masters, Mates and Pilots, the union that represents U.S. commercial ship officers, said the example of the Maersk Alabama should show the importance of the program.

"Unfortunately, we're under attack. What the pirates could not take away from the captain and his crew, the Congress could take away," he said. "The Maersk Alabama is one of the vessels that is part of the 60-ship Maritime Security Program."

"Everyone understands belt-tightening. It's just that we've had our belt tightened to the point where we could lose our pants," he said.

The Maritime Security Program run by the U.S. Department of Transportation spends $186 million annually to ensure the 60 ships are at the ready to carry cargo for U.S. troops at war and other government uses. The subsidy per ship is $3.1 million this year, according to congressional testimony.

Data released by the captains at a news conference said that during the wars in Iraq and Afghanistan, vessels enrolled in the security program carried 95 percent of Defense Department cargo shipped by sea to the region, from tanks to food.

The Maersk Alabama is one of the vessels potentially affected by the cuts.

Phillips was held hostage in a lifeboat after the pirates seized his vessel and attempted to hold it for ransom.

A five-day standoff with a U.S. Navy ship ended when snipers shot and killed Phillips' three captors on the lifeboat. "Captain Phillips," a movie about the incident starring Tom Hanks, opened in U.S. theaters this week.

Phillips said he didn't view himself as a hero and downplayed the incident, quipping: "We've always dealt with piracy. It's ... the second oldest profession we deal with in the Merchant Marine."