Japan's defense ministry reported the launch of what it said were two ballistic missiles by North Korea, both of which fell outside its exclusive economic zone.

The launches come amid heightened tensions on the Korean peninsula as South Korea and the US take steps to increase their military readiness against North Korea's weapons program with the deployment of US strategic military assets.

The US said it was consulting closely with its allies about the North Korean missile launches, which it described as being destabilizing.

Earlier on Monday, a nuclear-powered US submarine entered a naval base in South Korea's southern island of Jeju to load military supplies while on an unspecified operational mission, the South Korean navy said.

Over the weekend, the North fired a barrage of cruise missiles into the sea off its west coast.