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 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.

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

- '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.

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.

Seoul 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 Pyongyang would never accept.

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