North Korean leader Kim Jong-un has used his New Year address to claim all of the USA is within range of his country's nuclear missiles.

Key points: Kim Jong-un says North Korea is a peaceful nuclear nation and that the weapons will be used "only if our security is threatened"

Kim Jong-un says North Korea is a peaceful nuclear nation and that the weapons will be used "only if our security is threatened" North Korea conducted its sixth and most powerful nuclear test in September

North Korea conducted its sixth and most powerful nuclear test in September The comments came after Retired Admiral Mike Mullen said he believed the US has never been closer to a nuclear war with North Korea

"The US cannot declare war against us. The entire US territories are within our firing range and the nuclear missile button is right there on my desk," he said in a speech broadcast on state TV.

"We have secured powerful deterrence against the nuclear threat from the US," he added.

The North Korean leader said the US should be aware that his country's nuclear forces were now a reality not a threat, and North Korea achieved the historic feat of "completing" its nuclear forces.

"This year we should focus on mass producing nuclear warheads and ballistic missiles for operational deployment," Mr Kim said.

"These weapons will be used only if our security is threatened."

CNN reported Mr Kim said in his address that North Korea was "a responsible nuclear nation that loves peace".

North Korea tested intercontinental ballistic missiles and conducted its sixth and most powerful nuclear test in September in defiance of international warnings and sanctions, raising fears of a new conflict on the Korean peninsula.

Mr Kim said it was imperative to lower military tensions on the Korean peninsula and improve ties with the South, adding the path to dialogue was open.

Sorry, this video has expired Flight crews believe they saw the latest North Korean ballistic missile test.

He said he would consider sending a delegation to the Winter Olympic Games to be held in Pyeongchang, South Korea, in February.

South Korea 'welcomes' suggestion of Olympics meeting

The South Korean government welcomed the comment from Mr Kim.

South Korean presidential office spokesperson Park Soo-hyun told a news conference in Seoul that the Government hoped a solution could be found to ease tension on the peninsula and establish peace.

"The Blue House welcomes Chairman Kim Jong Un raising the necessity to improve the South-North Korean relationship, while mentioning the willingness to send North Korean athletes to the Pyeongchang Winter Olympics in his new year's speech and the suggestion that the two governments hold a meeting to discuss the issue," he said.

Mr Kim's comments came after a former chairman of the US Joint Chiefs of Staff said he believed the US had never been closer to a nuclear war with North Korea.

Retired Admiral Mike Mullen told ABC America's This Week program that "I don't see the opportunities to solve this diplomatically at this particular point".

"We're actually closer, in my view, to a nuclear war with North Korea and in that region than we ever have been," he said.

Mr Mullen also worried about China's involvement in restraining North Korea from making nuclear threats.

In November, US President Donald Trump asked China to cut off oil supply to North Korea to restrain the country from making the threats.

"I think President Trump has made China move more than they have in the past. Whether they continue to do that to help resolve this is the open question," he said.

"A real measure of how this all comes out is whether China is going to commit to a peaceful resolution here. If they don't, then I worry a great deal that it's much more likely there will be conflict."

Reuters/AP