Firing heavy machine-guns and mortars, U.S. soldiers practiced repulsing a commando attack Saturday at the maximum-security prison for terror suspects at Guantanamo Bay (search).

While the possibility of terrorists trying to break out prisoners seems remote, it's crucial for the soldiers to be prepared, said Capt. Gregg Langevin, a 33-year-old from the Massachusetts Army National Guard (search).

"There have been reports that the Al Qaeda are out there actively trying to buy small crafts," Langevin said, suggesting a stealthy approach from the coast.

Some 650 men from more than 40 countries are detained at the remote camp in eastern Cuba, suspected of fighting for Osama bin Laden or the ousted Afghan Taliban regime that sheltered his insurgents.

Maj. Gen. Geoffrey Miller, commander of the detention mission, said guards warned detainees that they would hear blasts that were part of a training exercise.

Tracer rounds glowed red against the Caribbean Sea as gunfire pattered the water and struck a floating metal target simulating a boat. Mortar shells exploded with thundering force, sending up puffs of smoke.

"No one can get in here — bottom line," Sgt. Johnny Saldana, a 30-year-old from Boston, said after firing hundreds of rounds from a .50-caliber machine gun.

Guantanamo is about 110 miles from Haiti, 150 miles from Jamaica and 230 miles from the U.S. coast around Miami.

The scenario of a hijacked airliner flying toward the base also was addressed, with some gunners shooting at puffs of smoke set off in the sky.

Other troops patrolled the rocky hills around the prison while some soldiers manned machine guns atop Humvees.

Medical teams practiced evacuating wounded to the base hospital, while a long line of cars and trucks formed on a nearby road where crews checked for bombs. Other security teams were tested by the appearance of a suspicious package, or an intruder attempting to film restricted areas.

Saturday's drills ended four days of exercises involving about 1,200 soldiers. Such training has taken place every four to six weeks since the prison camp was established in January 2002, in the wake of the Sept. 11, 2001 terrorist attacks and the war in Afghanistan.

Because escape from the base is an "enormously remote" possibility, that scenario isn't a training priority, Miller said.

Some liken the prison on the Caribbean island to a modern-day Alcatraz (search). Cellblocks are surrounded by double fences of razor wire and snipers in towers. Old Cuban minefields protect the base's boundaries.

So far the biggest threat has been security breaches. An Army intelligence officer, two Arabic translators and an Army Muslim chaplain have been charged with offenses ranging from attempted espionage to mishandling classified information.

Among the most serious charges are that an Air Force interpreter collected secrets about the base and messages from prisoners with plans to transmit them to an unspecified enemy in his native Syria.

Since his arrest as he prepared to board a plane to Damascus in July, security has been tightened, with new firewalls on computer systems and increased screening of bags and workers' electronic equipment.

The U.S. government holds the base under an open-ended 1903 lease. Although Fidel Castro's government staunchly opposes the U.S. presence, tension has eased since the 1960s, when at the height of the Cold War skirmishes erupted between U.S. and Cuban troops.

Now military officers from the two sides regularly discuss security, and U.S. troops face a more amorphous enemy.