Maj. Armando Fleites, military chief of the Escambray Second Front and Alpha 66, two anti-Castro groups, said the United States Coast Guard had prevented 12 men in two boats from going to Cuba today on a military mission.

Major Fleites said the men were intercepted about 75 miles south of Florida in the Florida Straits.

Three Coast Guard cutters participated in the seizure of one 45-foot boat and another 18-footer, Major Fleites said, and seized one 57-mm. cannon, several rifles and a "great amount of ammunition."

He said the men were taken to the Coast Guard station in Miami, where they were questioned and released.

The Coast Guard, asked to confirm the seizure, said, "That was a customs show." United States Customs officials had no immediate comment.