SEOUL - North Korea escalated its ongoing war of words with the United States on Monday (April 17), saying it would not curtail its missile-testing programme and warning of "all out war" if Washington took military action against it.

"We'll be conducting more missile tests on a weekly, monthly and yearly basis," Vice Foreign Minister Han Song-Ryol told the BBC's John Sudworth in an interview in Pyongyang. He went on to say that any US military action against North Korea would trigger an "all out war".

The warning in the interview came after US Vice-President Mike Pence arrived in South Korea and warned North Korea that his own country's "era of strategic patience" with Pyongyang was over.

Defying international pressure, the North on Sunday (April 16) test-fired another missile as fears grow that it may be preparing for its sixth atomic weapons test.

"We hope to achieve this objective (the North's denuclearisation) through peaceful means but all options are on the table," Pence told a press conference in the South Korean capital on Monday (April 17) after a trip to the tense border with the North.

"Just in the past two weeks, the world witnessed the strength and resolve of our new president in actions taken in Syria and Afghanistan.

"North Korea would do well not to test his resolve, or the strength of the armed forces of the United States in this region," Pence said at the press conference with South Korea's Acting President Hwang Kyo-Ahn.

Tensions between Pyongyang and Washington have soared in recent weeks, as a series of North Korean missile tests have prompted ever-more bellicose warnings from Trump's administration.

The new and inexperienced US president has indicated he will not allow North Korea to develop an intercontinental ballistic missile capable of delivering a nuclear warhead to the western United States.

North Korea's envoy to the United Nations said the regime was preparing for "any mode of war" triggered by potential US military action, and said his country would respond to a missile or nuclear strike "in kind".

"If the United States dares opt for a military action (...) the DPRK is ready to react to any mode of war desired by the Americans," Kim told a news conference, using the country's official name, the Democratic People's Republic of Korea.

"We will take the toughest counteraction against the provocateurs," he said.

Pence declared that North Korea had tested the world's patience for two decades and "answered our overtures with wilful deception, broken promises and nuclear and missile tests".

The US, which stations 28,500 troops in South Korea, would "defeat any attack and we will meet any use of conventional or nuclear weapons with an overwhelming and effective response".

Pence's trip earlier Monday (April 17) to the Demilitarised Zone between the two Koreas - one of the most heavily fortified frontiers on the planet - underscored Washington's changing policy towards the isolated state.

The visit came after a huge military parade Saturday during which North Korea showcased apparent intercontinental ballistic missiles, and as a US carrier group converges on the Korean peninsula.

It also came the day after North Korea's latest launch - which failed when the missile blew up seconds after blast-off.

North Korean state media last week warned of a nuclear attack on the United States at any sign of American aggression, but the White House said there was no evidence it possessed that capability.

The BBC reported Han also said North Korea believed its nuclear weapons protect it from the threat of military action by the United States.

Speaking at the village of Panmunjom inside the DMZ, Pence said America's relationship with South Korea was "ironclad and immutable".

Pyongyang insists it needs a powerful arsenal - including atomic weapons - to protect itself from what it says is the ever-present threat of US invasion.

A top White House foreign policy adviser on Sunday became the latest Trump official to warn that while diplomatic pressure was preferable, US military action is very much on the table.

Pence urged the international community to join US and regional demands for an end to the North's nuclear and ballistic missile programmes.

"It is heartening to see China commit to these actions. But the United States is troubled by China's economic retaliation against South Korea for taking appropriate steps to defend itself," he said, referring to the US THAAD missile defence system.

The system being installed in South Korea is designed to shoot down missiles from North Korea or elsewhere. But China furiously objects to its deployment, saying it could spy on its own defence installations, and has taken apparent retaliatory action against South Korean firms operating in its country.

Pence said he and Trump "have great confidence that China will properly deal with North Korea".

"But as President Trump made clear just a few short days ago, if China is unable to deal with North Korea, the United States and our allies will."

This is Pence's first visit to South Korea - part of an Asia swing that will also include stops in Japan, Indonesia and Australia - and although it was conceived months ago, could hardly come at a time of higher tension.