Secretary of State Mike Pompeo, left, shakes hands with Kim Jong-un in Pyongyang in May - KCNA via KNS

North Korea has ruled out dismantling its nuclear arsenal in exchange for the US declaring an end to the Korean War, saying on Tuesday that a peace treaty should "never be a bargaining chip."

The North has for decades demanded that the US formally declare the end of the 1950-53 conflict that was halted only with an armistice, saying an official end to the war would ease tensions on the flashpoint peninsula.

At a summit with the South's President Moon Jae-in last month, the North's leader Kim Jong-un offered to shut down its main Yongbyon nuclear complex if Washington takes "corresponding measures".

Kim did not elaborate on what those measure might be, but state news agency KCNA said in a commentary that some US experts have suggested trading the end of the war for denuclearisation.

It flatly described that as only the "most basic... process" to restore ties, not a point for negotiation.

"The end of war... is not just a gift from a man to another at all. Furthermore, it can never be a bargaining chip for getting the DPRK denuclearised," it said, using the North's full name.

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KCNA said that Pyongyang was willing to take "such... steps as eternal dismantlement" of its nuclear complex "if the US takes a corresponding measure" but again did not elaborate.

Cho Sung-ryul, analyst at the Institute of National Security Strategy, said the commentary may be aimed at limiting Washington's room for manoeuvre.

"The North is trying to reduce the negotiating value of Washington's potential offer of a peace treaty, by suggesting that 'it's not good enough to make us denuclearise'," he told South Korea's news agency Yonhap.

A landmark summit between Kim and US President Donald Trump in June led to a warming of ties, but there has been little concrete progress toward denuclearisation.

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The North's foreign minister Ri Yong Ho also told the UN last month that there was "no way" his country would disarm first as long as the US pushes for tough enforcement of sanctions against Pyongyang.

Trump however lauded Kim, saying last week the pair had fallen "in love" - their bromance fuelled by "beautiful letters" he recently received from the young leader.

The ceasefire that halted the bloody conflict and divided the Korean peninsula was signed by the North, its military ally China and the US - which fought on the side of the South.

The South did not take part as its then-leader refused to end the war unless the whole peninsula was unified.

On Tuesday a top South Korean official told MPs that North Korea is estimated to have up to 60 nuclear weapons, in Seoul's first public comment about the size of the North's secrecy-clouded weapons arsenal.

Unification Minister Cho Myoung-gyon told parliament the estimates on the size of North Korea's nuclear arsenal range from 20 bombs to as many as 60, saying the information came from the intelligence authorities. The National Intelligence Service, South Korea's main spy agency, did not immediately comment and it is believed Cho may have revealed the information unintentionally.

His ministry said on Tuesday that Cho's comments didn't mean that South Korea would accept North Korea as a nuclear state, suggesting Seoul's diplomatic efforts to rid the North of its nuclear program would continue.

The South Korean assessment on the North's arsenal is not much different from various outside civilian estimates largely based on the amount of nuclear materials that North is believed to have produced.

According to South Korean government reports, North Korea is believed to have produced 50 kilograms (110 pounds) of weaponised plutonium, enough for at least eight bombs. Stanford University scholars, including nuclear physicist Siegfried Hecker, who visited North Korea's centrifuge facility at Nyongbyon in 2010, wrote earlier this year that North Korea is estimated to have a highly enriched uranium inventory of 250 to 500 kilograms (550 to 1,100 pounds), sufficient for 25 to 30 nuclear devices.

Many foreign experts say North Korea is likely to be running additional secret uranium-enrichment plants.