North Korea fired about 10 ballistic missiles hours after Seoul put its nuclear arsenal at up to 120 warheads, twice the number US President Donald Trump claimed Pyongyang has earlier this week.

The missiles were launched from Pyongyang on August 20 and flew roughly 300 km, South Korea's Joint Chiefs of Staff said. Japan's coast guard said a suspected North Korean missile appeared to have fallen outside Japan's exclusive economic zone, according to public broadcaster NHK. US Pacific Command said the launches posed no immediate threat to American personnel, territory or allies.

However, the launch dramatically underlines the acceleration and escalation of an Asian nuclear arms race that is destabilising the Indo-Pacific region to the point where Japan has started to public debate the idea of building up its own nuclear arsenal.

Officials from the United States, South Korea and Japan held a three-way call the following day urging the North to stop "provocations that threaten the peace and security of the region and the international community", Japan's foreign ministry said.

The warhead dispute is the more consequential half of the week. "The numbers appear to be somewhat different," Defence Minister Ahn Gyu-back told lawmakers when asked about Trump's assessment that Kim Jong Un has "57 very powerful nuclear weapons".

"We usually look at around 80 to 120," Ahn said, later clarifying that he was citing a South Korean research institute rather than the military's own classified estimate, on which he said there were "considerable limitations" on what he could disclose.

The difference between 57 and 120 is the difference between US ground-based forces being able to defend themselves from a North Korean strike or not. US midcourse defence has 44 interceptors in Alaska and California with space for 20 more, at a cost of roughly $65bn - and standard doctrine fires at least two interceptors at each incoming warhead, which means the magazine empties against a couple of dozen intercontinental missiles.

Estimates of how many of those North Korea has vary wildly. The US Defence Intelligence Agency said last year it had only 10; Vann Van Diepen of 38 North puts the number of launchers as high as 48; Ankit Panda of the Carnegie Endowment for International Peace assesses up to 24 ICBMs with production continuing. Pyongyang has never demonstrated a large-scale coordinated launch.

"They have more experience stewarding their nuclear forces, they have far greater confidence in their delivery systems," Panda said at a recent seminar, which means "you have a nuclear-armed adversary in North Korea that's going to be far less skittish than they would have been a few years ago."

South Korean President Lee Jae Myung said in January that the North can now produce enough weapons-grade material for up to 20 weapons a year, a rate that would take it past Israel's stockpile and towards France's within a decade. Some arms experts put the output lower, at 12-15 bombs' worth a year - still faster than India, which added eight warheads between 2024 and 2025 to reach 180, according to the Stockholm International Peace Research Institute.

This month's launches were the third of August, and they came after Trump abruptly cancelled the decades-old annual military exercises with South Korea, a decision that stunned Seoul. Pyongyang responded by mocking the South as an American "puppet" and casting doubt on Trump's suggestion that high-level contact had resumed, saying that despite the "excellent" relationship between the two leaders, US hostile policy was unchanged. Lee, for his part, welcomed the cancellation as an opening.

The North has spent the past year making its posture harder to negotiate away, changing its constitution to mandate a nuclear strike if a decapitation attempt is made, and drawing closer to Russia, China and Iran as the war hardens the CRINK grouping into something like an alliance. All of it is happening with no arms control architecture left to hang a deal on.