Poder Cubano, a clandestine exile organization, Friday claimed responsibility for an explosion that severely damaged a Spanish freighter shortly after its departure from San Juan, Puerto Rico.

The FBI and Puerto Rican police attributed the explosion Thursday night to sabotage.

In a typewritten note sent to news media here, Poder Cubano said frogmen attached a bomb to the hull of the 5,228-ton freighter "Coromoto" directly under the engine room.

The blast caused severe damage to the interior of the vessel. There were no injuries.

The "Coromoto", sailing three miles off the Puerto Rican coast at the time of the blast, was towed back into San Juan.

A tight security guard was thrown around the vessel.

The vessel was en route to Santo Domingo and other Caribbean ports when the explosion occurred.

The freighter, operated by the Transatlantica Espanola Company, is a sister ship of the "Satrustegui", operated by the same company, which suffered bomb damage in San Juan harbor two years ago.

A Cuban exile living in San Juan was convicted of that sabotage.

The note from Poder Cubano said sabotage against the "Coromoto" was prompted by Spain's continuing trade with Cuba.

"Spain has joined the British, Canadians and Japanese," the note said, "among those who have already received what's coming to them from Poder Cubano."

Britain, Canada, Japan and Spain all trade with Cuba.

Poder Cubano has claimed credit for explosions aboard two Japanese and three British vessels, including the "Caribbean Venture" in the port of Miami last month. The group, claiming they operate outside U.S. territory only, apologized for that blast.

Earlier this week, Poder Cubano said it was responsible for an explosion and fire in a warehouse in Cuba that caused damages estimated by the Cuban government at $1.5 million.