Former North Korean soldiers who defected to South Korea on Monday claimed "underwater suicide squads" may have been responsible for the mysterious sinking of a South Korean naval vessel on Friday.

They are similar to the underwater demolition teams operated by the South Korean Navy, the defectors claimed. Recruited from the cream among North Korea's naval commandos, members of the teams are treated well but undergo brutal training.

According to one high-ranking North Korean defector, the North formed suicide attack squads in each branch of the military after the country's leader Kim Jong-il said during the U.S.-led invasion of Iraq in 2003 that no military in the world can defeat an army that can carry out suicide bombings.

The suicide attack squads are known as the "invincibles" in the Air Force, "bombs" in the Army and "human torpedoes" in the Navy. North Korea is said to place special emphasis on the naval squads. It operates a brigade of suicide attack squads in its East Sea and West Sea fleets and they are considered key to overcoming North Korea's inferior conventional military power.

One former North Korean sailor who defected to South Korea said the suicide squads have many semi-submersible vessels that can carry two bombers and either two torpedoes or two floating mines. In areas like the West Sea where the underwater current is fast, the suicide bombers train with mines rather than torpedoes.

One defector who served in North Korea's intelligence service, said, "Following the first naval battle in 1999, North Korea realized that it cannot defeat the South Korean Navy by conventional means and began studying unconventional methods." Th e best method is said to be the use of "acoustic mines" carried by small, semi-submersibles that travel at speeds of less than 2 km/h. The craft could be detected by South Korean sonar if they travel any faster. If the underwater squads returned after placing the mines on the hull of a ship, it would be very difficult to find evidence of the attack.