US prepared to attack North Korea if it tests nuclear weapon: reports

The US would be prepared to launch a preemptive strike on North Korea using conventional weapons if the rogue nation tested another nuclear device, several senior intelligence officials have reportedly told NBC.

North Korea told foreign journalists on Thursday to prepare for a "big event" this coming weekend with many analysts believing that could be a nuclear detonation.

On Thursday US President Donald Trump told reporters: "North Korea is a problem, the problem will be taken care of."

That could mean an attack on the North Korean nation and a conflict that could draw in US allies including Australia.

Unnamed intelligence personnel told NBC there were two US naval destroyers in the region, capable of firing Tomahawk cruise missile.

One of the destroyers is less than 500 kilometres from the possible test site in North Korea and there are heavy aerial bombers stationed at the US base island of Guam.

These forces are reportedly in addition to the battle group headed by the USS Carl Vinson aircraft carrier.

Tensions on the Korean peninsula have been cranking up after President Trump sent the "armada" into the region.

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That was followed by threats from North Korea that it would fire a nuclear strike on the US mainland with calls for China to step in and quell the country's nuclear ambitions.

On Thursday the US president tweeted that if China did not play its part, "the US, with its allies, will".

"I have great confidence that China will properly deal with North Korea," Mr Trump tweeted on Thursday.

"If they are unable to do so, the US, with its allies, will! U.S.A."

Mr Trump's inclusion of "allies" differs from a tweet he sent out on Tuesday.

"North Korea is looking for trouble," Mr Trump wrote.

At the same time Japan's prime minister warned of possible sarin nerve gas threats from the Hermit Kingdom, delivered by missile attack.

Prime Minister Shinzo Abe told parliamentary defence committee Thursday there was a "possibility that North Korea is already capable of delivering (via missile) a warhead containing sarin".

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The warning comes as Mr Trump publicly ramps up pressure on China to use its influence to curtail North Korean leader Kim Jong Un's nuclear and missile program.

North Korea has responded by threatening nuclear strikes on the US.

On Thursday Mr Trump told reporters China "has already started" asserting its influence on North Korea by turning back North Korean coal boats.

North Korean coal exports to China are vital for the already struggling regime's economy.

"The vast amount of coal that comes out of North Korea going to China, they've turned back the boats," Mr Trump told the joint press conference with NATO Secretary-General Jens Stoltenberg.

"That's a big step, and they have many other steps that I know about.

"So we'll see what happens.