The United States reaffirmed Friday a longstanding commitment to protect Japan through nuclear "deterrence" after talks that coincided with mounting threats from nuclear-armed North Korea.

The three days of discussions between US and Japanese diplomats and defense officials focused on "maintaining a credible deterrence posture in a changing security environment," said a Pentagon statement.

The meeting, part of what the Defense Department called a biannual "extended deterrence dialogue," was held at Naval Base Kitsap in Washington state.

Japanese officials were given tours of a naval submarine and Trident missile facilities, which form part of America's nuclear arsenal.

The talks are designed to make "clear to our allies that US extended deterrence continues to be credible, capable and enduring," the statement said.

As North Korea has progressed in its nuclear weapons program, South Korea and Japan have weighed developing their own capability but US officials have sought to reassure their allies that the American "triad" of nuclear-armed bombers, submarines and land-based missiles can counter potential threats.

The Pentagon statement made no explicit reference to North Korea.

Japan, meanwhile, vowed it would respond to "any scenario" after a threat by North Korea that Tokyo would be "consumed in nuclear flames."

North Korea is widely expected to launch medium-range missiles off its east coast in the run-up to April 15 national celebrations, in defiance of UN resolutions and international warnings.

Japan, the only country ever to have suffered a nuclear attack, has ordered its forces to shoot down any North Korean missile headed toward its territory.

Along with US military forces in the region, Japan and South Korea have bolstered missile defenses to prepare for a possible North Korean launch.