TOKYO — A Japanese warship accompanied a United States Navy supply ship on Monday on its way to join an American aircraft carrier and three other warships in a strike force that entered the Sea of Japan over the weekend.

The group is meant to send a powerful deterrent signal to North Korea at a time of mounting tensions on the Korean Peninsula over the North’s advancing nuclear program.

Japan’s action is a sign of its expanding military presence in the region. It is particularly significant because it represents the first time a warship is being used to aid an allied force since the country’s Parliament passed legislation authorizing overseas combat missions.

Prime Minister Shinzo Abe fought a hard political battle to push through those security laws two years ago, and they remain contentious in a country that has considered its postwar pacifism a deeply embedded part of its identity.