North Korea has officially enshrined the right to use preemptive nuclear strikes to protect itself in a new law that leader Kim Jong Un said makes its nuclear status "irreversible" and bars denuclearisation talks, its state media reported on Friday.

The secretary-general of the United Nations, which has long-standing sanctions on North Korea's nuclear and missile programs, said he was "deeply concerned" by the new law and reiterated calls for Pyongyang to return to denuclearisation talks.

The United States again said it had no hostile intent towards North Korea and is willing to resume talks without preconditions.

North Korea's move comes as observers say North Korea appears to be preparing to resume nuclear testing for the first time since 2017, after historic summits with then-US president Donald Trump and other world leaders in 2018 failed to persuade Kim to abandon his weapons development.

North Korea's rubber-stamp parliament, the Supreme People's Assembly, passed the legislation on Thursday as a replacement to a 2013 law that first outlined the country's nuclear status, according to the country's state news agency KCNA.

"The utmost significance of legislating nuclear weapons policy is to draw an irretrievable line so that there can be no bargaining over our nuclear weapons," it quoted Kim as telling the assembly. He added that he would never surrender the weapons even if the country faced 100 years of sanctions.

Among the scenarios that could trigger a nuclear attack would be the threat of an imminent nuclear strike; if the country's leadership, people or existence were under threat; or to gain the upper hand during a war.

White House press secretary Karine Jean-Pierre repeated past US statements that Washington has no hostile intent toward North Korea.

"As we have said, and North Korean officials, including Kim Jong Un, have publicly noted, we continue to seek diplomacy and are prepared to meet without preconditions. The DPRK continues to not respond," she said, using the initials of North Korea's official name.

"The United States remains focused on continuing to coordinate closely with our allies and partners to address the threats posed by DPRK," she added.

A deputy at the North Korean assembly said the law would be a powerful legal guarantee for consolidating North Korea's position as a nuclear weapons state and ensuring the "transparent, consistent and standard character" of its nuclear policy, KCNA reported.

Rob York, director for regional affairs at the Hawaii-based Pacific Forum, said spelling out the conditions for use of nuclear weapons was rare and showed how essential North Korea considered them to its survival.

Preemptive strikes

The 2013 law stipulated that North Korea could use nuclear weapons to repel invasion or attack from a hostile nuclear state and make retaliatory strikes.

The new law goes beyond that to allow preemptive nuclear strikes North Korea detects an imminent attack by weapons of mass destruction or of any kind aimed at its leadership and the command organisation of its nuclear forces.

That is an apparent reference to South Korea's "Kill Chain" strategy, which calls for preemptively striking North Korea's nuclear infrastructure and command system if an imminent attack is suspected.

Kim cited Kill Chain, part of a three-pronged military strategy being boosted under new South Korean President Yoon Suk-yeol, as a sign that the situation is deteriorating and Pyongyang must prepare for long-term tensions.

"In a nutshell, there are some really vague and ambiguous circumstances in which North Korea is now saying it might use its nuclear weapons," Chad O'Carroll, founder of the North Korea-tracking website NK News, said on Twitter.

"I imagine the purpose is to give US and South Korean military planners pause for thought over a much wider range of actions than before."

Under the law, Kim has "all decisive powers" over nuclear weapons, but if the command-and-control system is threatened, then nuclear weapons may be launched "automatically".

If Kim delegates launch authority to lower commanders during a crisis, that could increase the chances of a catastrophic miscalculation, analysts said.

'Responsible nuclear state'

KCNA said the new law bans sharing of nuclear arms or technology with other countries, and is aimed at reducing the danger of a nuclear war by preventing miscalculations among nuclear weapons states and misuse of nuclear weapons.

Analysts say Kim's goal is to win international acceptance of North Korea's status as a "responsible nuclear state."

Yoon has said Seoul would provide massive economic aid if Pyongyang began to give up its nuclear arsenal and Seoul on Thursday offered to hold talks with North Korea on reunions of families separated by the 1950-53 Korean War, in its first direct overture under Yoon, despite strained cross-border ties.

North Korea has rebuffed such overtures, saying that the United States and its allies maintain "hostile policies" such as sanctions and military drills that undermine messages of peace.

"As long as nuclear weapons remain on earth and imperialism remains and manoeuvres of the United States and its followers against our republic are not terminated, our work to strengthen nuclear force will not cease," Kim said.

Russia's Foreign Ministry said on Friday it was "closely monitoring" any military activity on the Korean peninsula.

North Korea's ally China did not address the new law and its implications when its Foreign Ministry was asked for comment on Friday, though Foreign Ministry spokesperson Mao Ning said its position on the Korean peninsula "had not changed".

China's policy on the Korean peninsula includes a long-standing commitment to the denuclearisation of North Korea.

"We will act in accordance with the broad framework of maintaining peace and stability on the (Korean) Peninsula," Mao said.

- Reuters