SEOUL North Korea fired two ballistic missiles into the sea off its east coast late on Monday, South Korea's military said, hours after a U.S. nuclear-powered submarine arrived in a naval base in the South.

Japan's defence ministry also 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 United States take steps to increase their military readiness against North Korea's weapons programme with the deployment of U.S. strategic military assets.

North Korea has reacted angrily, saying such a deployment could meet the criteria for its use of nuclear weapons.

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

In a statement on Monday, the U.S. military added the launches posed no immediate threat to U.S. personnel and territory or to U.S. allies.

Earlier on Monday, a nuclear-powered U.S. 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.

Last week, North Korea conducted ballistic missile tests after a nuclear-armed U.S. submarine arrived at a South Korean port for the first time since the 1980s.

(Reporting by Jack Kim in Seoul, Chang-Ran Kim in Tokyo and Kanishka Singh in Washington; Editing by Alison Williams, Andrew Heavens and Chris Reese)