The announcement comes at a time of crumbling ties between the North and South, with Pyongyang blaming Seoul for the outbreak of Covid-19 in its territory and conducting a record number of weapons tests this year.

The law will allow North Korea to carry out a preventive nuclear strike "automatically" and "immediately to destroy the hostile forces" when a foreign country poses an imminent threat to Pyongyang, the official Korean Central News Agency (KCNA) said.

With the newly enacted law, "the status of our country as a nuclear weapons state has become irreversible", leader Kim Jong Un said, according to KCNA.

Kim in July said his country was "ready to mobilise" its nuclear capability in any war with the United States and the South.

He reiterated that Pyongyang would never give up the nuclear weapons it needed to counter hostilities from Washington, claiming the United States sought to "collapse" his regime at any time.

Nuclear talks and diplomacy between Washington and Pyongyang have been derailed since 2019 over sanctions relief and what Pyongyang would be willing to give up in return.

"There is absolutely no such thing as giving up nuclear weapons first, and there is no denuclearisation and no negotiation," leader Kim said during a speech at North Korea's rubber-stamp parliament on Thursday, KCNA reported.

The new law shows Kim's confidence in his country's nuclear and military competence, including its intercontinental ballistic missiles capable of hitting the United States, said Cheong Seong-chang of the Center for North Korea Studies at the Sejong Institute.

The law "publicly justifies Pyongyang's use of its nuclear power" in the event of any military clash, including in response to non-nuclear attacks, Cheong told AFP.

- 'Height of absurdity' -

A blitz of North Korean weapons tests since January included the firing of an intercontinental ballistic missile at full range for the first time since 2017.

Washington and South Korean officials have repeatedly warned that the North is preparing to carry out what would be its seventh nuclear test.

Yang Moo-jin, a professor at the University of North Korean Studies, said the North's latest announcement clearly reaffirmed Pyongyang's stance -- that nuclear negotiations are no longer on the table.

"Pyongyang is likely to form closer ties with China and Russia against Washington, and ... launch its seventh nuclear test in the near future," he told AFP.

Seoul, Washington's key security ally, last month offered Pyongyang an "audacious" aid plan that would include food, energy and infrastructure help in return for the North abandoning its nuclear weapons programme.

But Pyongyang ridiculed the offer, calling it the "height of absurdity" and a deal the North would never accept.

South Korea's hawkish President Yoon Suk-yeol said last month that his administration had no plans to pursue its own nuclear deterrent.