Despite increasing violence in Central America, the countries of this region are not used to having their seaports mined. So when the Panamanian freighter Los Caribes saw the flashing channel buoy outside the Nicaraguan port of Corinto on March 7, it steamed right ahead.
At 11:15 p.m. a blast rocked the ship in the channel two miles from the port, the crew said.
''We felt a shock that lifted us about a meter and a half off the deck,'' said Gustavo Ulloa, third officer, quoted in the Nicaraguan government newspaper Barricada.
Nicaraguan rebels, armed and largely funded by the United States, later claimed responsibility for the act. Eleven such explosions reported in the past 40 days appear to mark an escalation of the rebel war against the Sandinista government.
A run-in between the Soviet Union and US followed when the Soviet oil tanker Lugansk struck a mine in Puerto Sandino March 20. The Soviets accused the US and its rebel allies of ''banditry and piracy.'' The US rejected the allegation that it was involved, although it said anti-Sandinista forces had widely advertised that some Nicaraguan ports were mined. The US countercharged that the Soviets were encouraging conflict in the region and helping to increase the level of violence there.
Meanwhile the toll of injury - physical and monetary - is mounting.
Three men were injured in the Los Caribes explosion and the ship suffered a hole in its hull. The vessel was towed into port, and its holds were pumped continuously while stevedores battled to save its cargo of machinery.
In all, two men have died and at least 20 others have been injured in 11 reported explosions in Nicaragua's Atlantic and Pacific ports in the past 40 days, according to the Sandinista government.
The effects of the mining are being felt in the Nicaraguan economy as well. Three times in the past month, ships destined for Corinto refused to enter the port and instead unloaded their cargo in the Costa Rican port of Caldera, down the Pacific coast, says Francisco Martinez, administrator of the Port of Corinto.
In addition, Nicaragua is just ending its coffee and cotton harvests and is sending these two principal export crops to port for harvest. But Nicaraguans are worried that it will have difficulty loading ships.
''This is our peak export season and the imperialists know that,'' says Mr. Martinez.
There is no sign the situation will improve soon.
''The ports will continue to be mined as long as the communist Sandinista regime illegally holds power,'' a leader of the rebel organization Nicaraguan Democratic Force said last week in Tegucigalpa, the capital of Honduras.
The Sandinistas went to the United Nations Security Council last week with charges that the United States was promoting ''terrorist acts against economic and military installations'' in Nicaragua. They charge that a US warship had been spotted off the coast about the time the Soviet ship hit a mine, and that high-powered ''piranha'' speedboats, operating from the US ship, had placed the mines near the port.
Defense Minister Humberto Ortega Saavedra has also gone to Moscow, asking for more military hardware, including at least one minesweeper to patrol the country's ports, according to diplomatic sources.
The Nicaraguans' primitive minesweeping - which includes use of fishing boats dragging weighted nets through the channel - has not eliminated the mines.
According to Army chief of staff Joaquin Cuadra, cited in Barricada, three government fishing boats have been destroyed by mine explosions while they completed patrol duties in the channel.
And last Friday, the Japanese freighter Terushio Maru, due to load some 13, 000 bales, hit a mine as it entered the port. Damages were light and there were no injuries reported. Once again, according to military authorities, a US warship was nearby.
''We saw it with radar, 23 to 27 miles from the port,'' said Capt. Mario Aleman. He said he believed ''piranhas'' had been lowered from the ship and had placed the mine the ship hit.
Martinez says he had assured the owners that tugboats or state-owned fishing boats would risk themselves by passing through the channel first, but that offer had been refused.
The Japanese ship was taking on cotton Tuesday, according to port officials, but the Nicaraguans are worried.
Lloyds of London, which insures many of the world's merchant ships and much of their cargo, has not declared Nicaragua a theater of war and has not raised its insurance rates for ships entering Nicaraguan waters, according to local shipping agents.
A rate hike would eventually result in a similar raising of shipping costs. This would raise the price of goods coming into the country and lower profits on Nicaragua's exports, local shippers say.