The Pentagon has a sketchy contingency plan for deploying ships and aircraft to stop war supplies from entering and leaving Nicaragua by sea and air.

But imposing such a quarantine, under which some ships would be stopped by U.S. ships and others allowed to pass, or a blockade, which can mean sealing off an entire coast from any shipping, is no easy task, even when applied to a small nation such as Nicaragua.

The plan calls for stationing one U.S. aircraft carrier and its protecting ships and submarines off Nicaragua's east coast and a second carrier off its west coast. This would require the Navy to leave other areas of the world thinly covered and use millions of dollars that otherwise might go for new weaponry.

Running one Nimitz-class nuclear aircraft carrier for one day, including flight operations, costs $501,308, according to the Navy. A carrier requires escorts of as many as six warships and two submarines, greatly adding to the cost of deploying such task forces on both coasts of Nicaragua.

Another problem is the difficulty and cost of keeping such task forces supplied, especially when deployments last months rather than days. Nicaragua is far from Navy supply centers in Puerto Rico and California.

Cuban President Fidel Castro is likely to oppose using the U.S. base at Guantanamo, Cuba, to support U.S. efforts to interdict war supplies bound for Nicaragua. His navy could also increase the risk of imposing a blockade by harassing supply ships sailing from ports in Puerto Rico.

Although the U.S. Navy could blockade Nicaragua despite these and other difficulties, military planners note that the effort would not stop war supplies from moving overland from Nicaragua through Honduras to guerrillas in El Salvador.

Such a "land bridge" is difficult to close without putting troops on the ground, as demonstrated during the Vietnam war when U.S. air power could not cut off the flow of supplies from North Vietnam to South Vietnam.