North Korean leader Kim Jong Un on Wednesday said his country would continue developing nuclear programmes and introduce a "new strategic weapon" in the near future, state media KCNA said after the United States missed a year-end deadline for a restart of denuclearisation talks.

As the US makes "gangster-like demands" including continuing joint military drills with South Korea, adopting cutting-edge weapons and imposing sanctions, there were no grounds for North Korea to be bound any longer by the self-declared nuclear and inter-continental ballistic missile test moratorium, Kim said, according to KCNA.

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Kim convened a rare four-day meeting of the ruling Workers' Party's policy-making committee, started since Saturday, as the US had not responded to his repeated calls for concessions to reopen negotiations, dismissing the deadline as artificial.

He pledged to further develop North Korea's nuclear deterrent, but left the door open for dialogue, saying the "scope and depth" of that deterrent will be "properly coordinated depending on" the attitude of the US.

"The world will witness a new strategic weapon to be possessed by the DPRK in the near future," Kim said, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.

"We will reliably put on constant alert the powerful nuclear deterrent capable of containing the nuclear threats from the US and guaranteeing our long-term security."

Hours after Kim's statement, US President Donald Trump told reporters said Kim had signed a contract about denuclearisation and that he thought the North Korean leader was a "man of his word."

Trump said he got along with Kim and "we have to do what we have to do."

"But he did sign a contract, he did sign an agreement talking about denuclearisation. ... That was done in Singapore, and I think he's a man of his word, so we're going to find out," he added.

US Secretary of State Mike Pompeo said he hoped North Korea would "choose peace and prosperity over conflict and war."

Kim had previously said he might have to seek a "new path" if Washington failed to meet his expectations.

US military commanders said Pyongyang's actions could include the testing of an intercontinental ballistic missile (ICBM), which it has halted since 2017, alongside nuclear warhead tests.

'Strategic weapons'

Tension had been rising in the run-up to the year-end deadline as North Korea conducted a series of weapons tests and waged a war of words with Trump.

Kim has called for active 'diplomatic and military countermeasures' to preserve the country's security amid US sanctions and threats [Ahn Young-joon/AP]

The nuclear talks have made little headway though Kim and Trump have met three times.

A working-level meeting in Stockholm in October fell apart, with a North Korean chief negotiator accusing US officials of sticking to their old stance.

KCNA quoted Kim as saying that there would "never be denuclearisation on the Korean Peninsula" if Washington adhered to what he called its hostile policy.

We "will steadily develop necessary and prerequisite strategic weapons for the security of the state until the US rolls back its hostile policy towards the DPRK and lasting and durable peacekeeping mechanism is built," Kim said.

He called for his people to brace for an "arduous and prolonged struggle" and foster a self-reliant economy because of delays in a much-anticipated lifting of sanctions.

"The present situation warning of long confrontation with the US urgently requires us to make it a fait accompli that we have to live under the sanctions by the hostile forces in the future, too, and to strengthen the internal power from all aspects."

Harry Kazianis, senior director of Korean Studies at the Centre for the National Interest in Washington, said Kim appeared to be gambling that threatening another demonstration of his ability to hit the US with a nuclear weapon would somehow push the US into granting more concessions.

"North Korea has, in effect, put an ICBM to Donald Trump’s head in order to gain the two concessions it wants most: sanctions relief and some sort of security guarantee,” he said.