Like their comrades nearby in the Persian Gulf, U.S. sailors will soon begin searching suspicious merchant vessels off the coast of Pakistan. But the contraband they're seeking may shoot back.

Pentagon officials announced Wednesday that the Navy would board cargo ships suspected of carrying Usama bin Laden or other Al Qaeda leaders who might flee Afghanistan by sea. No such boardings have yet taken place, said Marine Corps Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff.

A boarding operation might begin with U.S. intelligence operatives learning of an attempt by an Al Qaeda leader to escape from a port on the coast of Pakistan or Iran, military officials said.

If a ship is targeted, one or more of the 5th Fleet warships in the area would move to intercept it, said Lt. Col. Dave Lapan, a Pentagon spokesman. The U.S. Navy has assembled a massive fleet about 100 miles off the Pakistani coast, built around three aircraft carriers and two Marine amphibious assault ships.

The fleet's escorts, including missile-armed frigates, destroyers and cruisers, are the best bet to conduct such interdiction missions. They can fight surface battles and have crew members trained for boarding operations. Many have experience boarding ships suspected of smuggling Iraqi oil in the Persian Gulf.

The warship would send a radio hail or signal flashes to the suspicious vessel's captain, followed by orders to stop and prepare for boarders, Lapan said.

If the vessel did not heave to, the U.S. warship would fire a warning shot from its main gun across the bow of the fleeing ship. Armed helicopter gunships could be dispatched, ready to pounce. If the ship continued to resist, the warship could attack it.

Once the freighter or tanker is stopped, a boarding party, usually a team of armed U.S. sailors and Coast Guard personnel, would cross the space between the large ships on a small boat or a helicopter.

"The team is most vulnerable just as they're trying to board the vessel," said retired Adm. Stan Arthur, who commanded the naval vessels in the Persian Gulf during the Gulf War.

If the boarding was not resisted, the team would begin searching the ship, seeing if its papers are in order and verifying it is carrying the cargo listed on its manifest. If the target is a person, the team would begin a hunt through dark passageways, knowing they may face deadly close-quarters combat with terrorists who don't want to be taken alive.

"It's hard work on the young folks that do that," Arthur said. "The anxiety level is high. There is a lot of climbing and crawling around the ships."

The danger isn't just from unfriendlies on board, as the sailors who have been conducting interdiction in the Persian Gulf have learned. Two U.S. sailors from the destroyer USS Peterson have been missing since Sunday in the Persian Gulf after boarding a freighter that was smuggling oil from Iraq. The decrepit freighter sank while the U.S. party was on board.

For more than a decade, the Navy has stopped ships in the Persian Gulf to prevent Iraq from getting income from oil sales. Arthur recalled only one brief exchange of gunfire between U.S. troops and a merchant ship's crew, and he said the incident didn't escalate into a full-blown firefight.

Gen. Pace said the military has not learned of any specific attempts by terrorists to flee the region by ship.

"The amount of real estate available in Afghanistan for the Al Qaeda leaders to feel safe is being reduced, and as a precautionary measure, as you would expect us to, we are looking at how would they might try to flee the country," he said.

If bin Laden tries to flee by ship, he would have to leave landlocked Afghanistan and cross Pakistan or Iran to reach the sea.

Pakistan has very few natural harbors, but the thinly populated coast is a paradise for smugglers. The country's only major port is Karachi, which is also home to several hardline Islamic parties that support the Taliban and bin Laden. Allies there could help him slip across the border in Balochistan province near Quetta, where there are sympathetic tribes that could conceivably help him reach Karachi undetected.

Two more towns, Omara and Pasni, with small harbors could conceivably be reached overland from the Afghan border. They could not take large ships but could be used by dhows, small wooden boats that have been smuggling to and from the Persian Gulf for over a thousand years. Those small boats could carry bin Laden to a larger ship waiting off the coast.

Iran, which has little love for bin Laden, is considered a less likely avenue for escape.

It's unclear if warships belonging to U.S. allies will participate in the interdiction effort, or if Pakistan or Iran's coastal navies will support the effort. It's also unclear if the U.S. Navy will be allowed to operate in territorial waters of either country. While Pakistan's government has embraced the U.S. effort in Afghanistan, Iran has offered limited support.