The Maritime Self-Defense Force helicopter carrier Izumo is seen on its way to guard a U.S. Navy supply ship in the Pacific Ocean, in the sea east of Izu Oshima Island in this photo taken from a Mainichi helicopter shortly before 2 p.m. on May 1, 2017. (Mainichi)

Japan launched a mission to guard a U.S. military vessel on May 1 for the first time under security legislation that came into force in March 2016, sources close to the government said.

The Defense Ministry dispatched the Maritime Self-Defense Force (MSDF) helicopter carrier Izumo on May 1 to guard a U.S. Navy supply ship in the Pacific Ocean.

According to the sources, the Izumo joined the U.S. supply vessel off the Boso Peninsula later in the day, and began to guard the U.S. Navy ship.

The move is aimed at demonstrating the close Japan-U.S. alliance and deepening cooperation between the Self-Defense Forces (SDF) and U.S. forces amid rising tensions over the North Korean situation.

Regarding new SDF missions under the security legislation, Ground Self-Defense Force (GSDF) personnel dispatched to participate in U.N. peacekeeping operations in South Sudan were authorized to carry out a "rush-and-rescue" mission to rescue U.N. and NGO officials or foreign military personnel under attack. However, the GSDF personnel ended their mission and came home without carrying out the operation.

Therefore, the MSDF's guarding of the U.S. military vessel in the Pacific Ocean is the first new mission that the SDF has actually carried out under the new legislation.

The MSDF's Izumo left the force's Yokosuka base in Yokosuka, Kanagawa Prefecture, at around 10 a.m. on May 1. According to sources, the MSDF vessel is sailing side-by-side with the U.S. supply ship off the Pacific coast toward Shikoku. Minimal use of weapons by MSDF personnel aboard the ship would be permitted should either of the two vessels come under attack.

The Japanese and U.S. ships will subsequently head toward Singapore. The U.S. supply ship may refuel U.S. military vessels on alert against North Korea, such as the USS Carl Vinson nuclear-powered aircraft carrier that has been deployed in the Sea of Japan.

The SDF is allowed to guard U.S. and other military vessels engaged in activities that contribute to the defense of Japan under the revised SDF Act.

The guidelines the government has worked out for such operations assume that Japan can guard U.S. and other military vessels engaged in monitoring while on alert against the possible launch of ballistic missiles, gather relevant information, and carry out refueling and transportation operations in contingencies that could develop into armed attacks on Japan if left unaddressed, as well as joint drills.

Since the MSDF vessel and personnel are engaged in a mission in the Pacific Ocean away from North Korea, the Japanese troops are apparently guarding the U.S. supply ship while conducting a joint drill.

Some U.S. military vessels including an Aegis destroyer have been deployed to waters off the Korean Peninsula to be on alert against possible missile launches by North Korea.

The missions could help deepen integration between the SDF and U.S. forces.