A Maritime Self-Defense Force vessel conducted an exercise on Wednesday to protect a U.S. Navy ship in Japanese waters — an operation enabled by Japan's security legislation that took effect in March last year, the MSDF said.

It is the first time that the MSDF has announced training with the United States for the protection of a U.S. ship. The MSDF engaged in a real mission in May, escorting a U.S. naval ship out of Japanese waters, but Tokyo had not previously made the training public out to avoid any potential negative impact on U.S. military activities and the bilateral relationship.

Wednesday's drill took place during heightened tensions in the region over North Korea's nuclear and missile threats. MSDF and U.S. Navy personnel — aboard the minesweeper tender Bungo and the minesweeper Pioneer, respectively — checked communication procedures for a simulated protection mission at Mutsu Bay in Aomori Prefecture.

"The aim was to improve the MSDF's tactical skills and strengthen ties with the U.S. Navy," the MSDF said in a press release.

On May 1, an MSDF vessel engaged in the first-ever mission to protect a U.S. ship, as the helicopter carrier Izumo accompanied a U.S. Navy supply vessel sailing in waters off Japan's Pacific coast, Kyodo News and other media outlets reported.

Japan's security legislation has loosened the constraints the Constitution had previously imposed on activities of the nation's defense forces. The Self-Defense Forces are now 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, such as when the two countries are conducting joint exercises or monitoring and information-gathering activities related to North Korean missile launches.