The United States will counter any North Korean attack with an "overwhelming and effective" response, Vice President Mike Pence vowed Wednesday, as he stood on the deck of a massive American aircraft carrier docked in Japan.

Donald Trump's deputy is in the region to reassure allies fretting over Pyongyang's quickening missile programme, and its apparent readiness to carry out another banned nuclear test in its quest to develop an atomic weapon that can hit the US mainland.

Pence, whose visit started in South Korea the day after the failed launch by North Korea of what analysts said could have been a new missile, said the threat from the isolated regime was growing.

Aboard the USS Ronald Reagan, the vice president told troops he was there as "storm clouds gather on the horizon" of Northeast Asia.

"North Korea is the most dangerous and urgent threat to peace and security in the Asia-Pacific," Pence said. 

But, "we will defeat any attack and meet any use of conventional or nuclear weapons with an overwhelming and effective American response."

Pence's comments come after a senior North Korean official warned the regime had no intention of dialling down its missile programme, pledging weekly tests and threatening "all-out war" if the US took any action against it.

That kind of rhetoric has unnerved allies in Japan and South Korea, who would be at the sharp end of any North Korean response.

Seoul, the South Korean capital, is just a stone's throw away from the military demarcation line that splits the Korean peninsula and is within easy range of North Korean long-range artillery.

The Ronald Reagan, whose home port is Yokosuka, Japan, is part of the Seventh Fleet and is regularly deployed around the western Pacific. 

Another part of the Seventh Fleet, the USS Carl Vinson, is expected to be sent to the Sea of Japan (East Sea) as a show of force against the North Korean regime.

- Treaty allies -

Wearing a green flight jacket, Pence also sought to reassure jittery allies -- who have seen Trump call into question decades-old mutual defence treaties -- of America's commitment.

The United States, Pence said, would be unwavering in its alliances and unyielding in its resolve.

In a veiled warning to China, Pence added that "our treaty (with Japan) covers all the territory administered by Japan, including the Senkaku Islands" -- a disputed archipelago in the East China Sea that is controlled by Japan but claimed by Beijing.

There was also a warning to China regarding the South China Sea, where Beijing has built reefs and islets into fortified islands capable of hosting military assets to bolster its claim to sovereignty over the sea.

Pence said the United States would defend the right to freedom of navigation through the waterway, one of the most important shipping channels on the planet.

He also vowed that more of America's most advanced military assets would be deployed to the Asia-Pacific.

About 47,000 US troops are stationed in Japan and a further 28,000 are in South Korea.

Source: AFP