A Department of Defense release in Ottawa said the incident happened Tuesday morning when the HMCS Calgary received a garbled distress message from an unidentified cargo ship, the Canwest News Service said Wednesday.

The navy frigate altered course and dispatched a Sea King helicopter, the release said. The helicopter crew reported sighting two small boats with armed men aboard near the ship, about 90 miles off the Somali coast, the report said.

The small boats sped away when the helicopter was seen, said the Calgary's commander, Kelly Larkin.

"I am convinced that the presence of our aircraft drove them away from the traffic lanes and prevented any further attack today on merchant shipping in the area," Larkin said.

The International Maritime Bureau said six attacks, including two hijackings, have been reported by vessels in the gulf since May 19, the report said.