Japan has dispatched its biggest warship, in the first of such operation since it passed controversial laws expanding the role of its military.
The helicopter carrier Izumo is escorting a U.S. supply vessel within Japanese waters.
The U.S. ship is heading to refuel the Naval fleet in the region, including the Carl Vinson aircraft carrier group.
North Korea has threatened to sink the Carl Vinson and a U.S. submarine, amid rising tensions in the region.
It also carried out a failed missile test on Sunday, despite repeated warnings from the U.S. and others to stop its nuclear and missile activity.