In Seoul US defence secretary said Pyongyang engages in ‘outlaw’ behaviour and that the US will never accept a nuclear North Korea.

US defence secretary Jim Mattis has said the threat of nuclear missile attack by North Korea is accelerating.

In remarks in Seoul with South Korean defence minister Song Young-moo at his side, Mattis accused the North of illegal and unnecessary missile and nuclear programs – and vowed to defeat any attack.

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Mattis said the North engages in “outlaw” behaviour and that the US will never accept a nuclear North.

He added that regardless of what the North might try, it is overmatched by the firepower and cohesiveness of the decades-old US-South Korean alliance.

“North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs,” he said, adding that US-South Korean military and diplomatic collaboration thus has taken on “a new urgency.”

“I cannot imagine a condition under which the United States would accept North Korea as a nuclear power,” he said

As he emphasized throughout his weeklong Asia trip, which included stops in Thailand and the Philippines, Mattis said diplomacy remains the preferred way to deal with the North.

“With that said,” he added, “make no mistake – any attack on the United States or our allies will be defeated, and any use of nuclear weapons by the North will be met with a massive military response that is effective and overwhelming.”

Mattis’s comments in Seoul did not go beyond his recent statements of concern about North Korea, although he appeared to inject a stronger note about the urgency of resolving the crisis.

Quick guide Are US defences strong enough to ward off North Korean missiles? Show Hide What kind of anti-missile defences does the US possess? The US has various anti-missile options, some designed to take down missiles at short-range and others for medium-to-long-range. The US relies heavily on the US Patriot missile and the Terminal High-Altitude Area Defence (THAAD). The US deployed THAAD to South Korea this year to defend against medium-range missiles. There is a three-phased defence system: ground-based missiles on the Korean peninsula; US naval ships stationed in the Pacific; and two bases in Alaska and California that can launch an estimated 36 interceptors.

Is the US system robust enough to stop a North Korean missile attack? No air defence system offers anything like a complete guarantee of success. The Pentagon offer repeated assurances that air defence systems would be more than a match for any North Korean attack. But when missile defence systems have been put to the test over the last few decades, the performance has been far from reassuring.

The US provided anti-missile defence systems to Israel and Saudi Arabia during the First Gulf War as protection against Iraq's Scud missiles. It was initially claimed that they had shot down 41 of 42 missiles fired by Iraq. But eventually it was acknowledged that only a few missiles had been hit.

Recent tests of interceptors have provided little comfort – with success rates of around 50% on average. The Pentagon celebrated in May when it destroyed a mock warhead over the Pacific but overall the performance has been spotty. Since the newest intercept system was introduced in 2004 only four of nine intercept attempts have been successful. Of the five tests since 2010, only two have been successful.

While he accused the North of “outlaw” behaviour, he did not mention that President Donald Trump has ratcheted up his own rhetoric. In August, Trump warned the North not to make any more threats against the United States, and said that if it did, it would be met with “fire and fury like the world has never seen.”

Song, the South Korean minister, told the news conference that he and Mattis agreed that limits on South Korea conventional missile warhead payloads would be lifted. He offered no specifics.

Also discussed were the conditions under which South Korea would be given wartime operational control of its forces. Currently, if war with the North broke out, the South’s forces would operate under the US-led UN Command.

Trump entered office declaring his commitment to solving the North Korea problem, asserting that he would succeed where his predecessors had failed. His administration has sought to increase pressure on Pyongyang through UN security ouncil sanctions and other diplomatic efforts, but the North hasn’t budged from its goal of building a full-fledged nuclear arsenal, including missiles capable of striking the US mainland.

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If Trump sticks to his pledge to stop the North from being able to threaten the US with a nuclear attack, something will have to give – either a negotiated tempering of the North’s ambitions or a US acceptance of the North as a nuclear power.

The other alternative would be US military action to attempt to neutralize or eliminate the North’s nuclear assets – a move fraught with risk for South Korea, Japan and the United States.

At his Seoul news conference, Mattis said the North is, in effect, shooting itself in the foot.

“If it remains on its current path of ballistic missiles and atomic bombs, it will be counterproductive, in effect reducing its security,” he said.

The North says it needs nuclear weapons to counter what it believes is a US effort to strangle its economy and overthrow the Kim government.