A Japanese Maritime Self-Defense Force helicopter carrier joined a U.S. Navy vessel in the Pacific on Monday for the first protection mission under security legislation that took effect last year, with tension running high in the region over North Korea.

The mission, being conducted by the helicopter carrier Izumo, Japan's largest postwar naval vessel, is apparently aimed at demonstrating the robust Japan-U.S. security alliance and deterring North Korea from conducting nuclear and missile tests.

Prime Minister Shinzo Abe's government has expanded the role of the Self-Defense Forces through the security legislation to more actively contribute to regional and global peace, but the move has been controversial in the country, where many people cherish the pacifism embedded in the post-World War II Constitution.

Some citizens protested against the mission near Yokosuka base, southwest of Tokyo, from where the Izumo departed Monday morning to join the U.S. Navy supply ship off the Boso Peninsula in Chiba Prefecture, east of the capital.

The 19,500-ton Izumo, which is equipped with a large flight deck, is expected to sail for two days in the Pacific toward waters off the Shikoku region in western Japan, according to government sources.

The supply ship escorted by Izumo is likely to refuel other U.S. vessels currently deployed in waters near Japan in anticipation of possible further missile test-firings by North Korea, as well as ships sailing with the U.S. Navy's aircraft carrier Carl Vinson that entered the Sea of Japan on Saturday.

Having left Singapore on April 8 to head to waters near the Korean Peninsula, the Carl Vinson has actively engaged in drills with Japan and South Korea. It conducted bilateral exercises with the MSDF in the Philippine Sea on April 23 and in the Sea of Japan on Saturday.

Once the protection mission ends, the Izumo is expected to take part in an international fleet review organized by the Singapore Navy.

Under Japan's security legislation, which loosened constraints imposed on SDF activities by the war-renouncing Constitution, SDF personnel are allowed to guard vessels and weapons belonging to U.S. forces when the latter are engaged in activities beneficial to the defense of Japan.

The protection mission can be conducted in various situations, including when the two countries are engaged in joint exercises or monitoring and information-gathering activities related to North Korean missile launches.

The MSDF personnel are allowed to use weapons to a certain extent. But due to constitutional constraints, they must stay away from areas where combat activities are taking place involving the armed forces of a foreign country to which Japan is providing support.

As the Constitution bans the "use of force" by Japan to settle international disputes, the government restricts the area of SDF activities to ensure they will not be viewed as an "integral part" of the military operation conducted by other countries.

Before the security legislation took effect in March last year, the SDF were able to use weapons to defend Japanese vessels and ammunition, but not the equipment owned by other countries.

The legislation has brought other changes to SDF activities, including during U.N. peacekeeping operations.

A batch of Ground Self-Defense Force personnel had been ordered to perform new responsibilities during a U.N. peacekeeping mission in South Sudan, including rescuing U.N. staff under attack. But the troops are leaving the African country in stages without actually engaging in the new roles as the Japanese government decided in March to withdraw the GSDF troops.