Relations between the two Koreas are at one of their lowest points ever, with diplomacy stalled and North Korean leader Kim Jong Un calling for increased weapons development, including tactical nuclear weapons.

The United States and South Korea have stepped up displays of military muscle in response, and a US nuclear-capable submarine made a port call to South Korea this week for the first time since 1981.

Pyongyang's defence minister Kang Sun Nam said the arrival in Busan port of a US Ohio-class submarine "may fall under the conditions of the use of nuclear weapons specified in the DPRK law on the nuclear force policy", referring to North Korea by its official name.

North Korea last year adopted a sweeping nuclear law, setting out an array of scenarios in which it could use its nukes, including pre-emptive nuclear strikes if threatened.

Kang said the presence of the sub was an "undisguised and direct nuclear threat to the DPRK", and meant that "strategic nuclear weapons have been deployed on the Korean peninsula for the first time after 40 odd years".

"The US military side should realise that its nuclear assets have entered extremely dangerous waters," he said in a statement carried by the official Korean Central News Agency.

South Korean President Yoon Suk Yeol visited the Ohio-class submarine Wednesday, and warned Pyongyang that were it to use nuclear weapons against the South, "it will lead to the demise of its regime".

Ohio-class subs can carry up to 20 Trident II ballistic missiles. The US Navy typically does not confirm if a submarine is carrying nuclear weapons before it goes out to sea.

The allies also held their first Nuclear Consultative Group meeting in Seoul Tuesday, to improve their joint response to any nuclear attack by North Korea.

- 'Direct, outright threat' -

It appears that the deployment of the submarine has rattled the North, Hong Min, a researcher at the Korea Institute for National Unification, told AFP.

"Pyongyang's message is that the US and South Korea are acting in a manner that is bringing the North closer to consider invoking its nuclear weapons use guidelines," he said.

"Yoon's comment during his visit to the submarine mentioning the end of the North Korean regime appears to have rattled Pyongyang a lot, prompting it to react much more aggressively."

North Korea has conducted a string of banned weapons tests this year, including twice launching its newest solid fuel Hwasong-18 intercontinental ballistic missile.

Seoul and Washington have ramped up defence cooperation in response, staging joint military exercises with advanced stealth jets and US strategic assets.

Washington first announced it would deploy a submarine capable of launching ballistic missiles with nuclear warheads to the Korean peninsula in April, while Yoon was on a state visit.

North Korea baulks at having US nuclear assets deployed around the Korean peninsula.

Kang said that for Washington and Seoul, "any use of their military muscle against the DPRK will be their most miserable choice by which they will have no room to think of their existence again".

Pyongyang's statement comes as an American soldier, Travis King, is believed to be in North Korean custody after crossing the border during a tourist trip to the Joint Security Area in the Demilitarised Zone on Tuesday.

Pyongyang has a long history of detaining Americans and using them as bargaining chips in bilateral ties. It has not yet issued any comment on King.

The US-led United Nations Command has said it is working with North Korea's military to "resolve this incident".

But with relations between Pyongyang and Washington at one of their lowest points in years, experts say it will be difficult to gain consular access to King.

In addition, nearly all foreign embassies in Pyongyang withdrew foreign staff after North Korea closed its borders in 2020 due to the Covid-19 pandemic.

That includes Sweden, which handles US consular affairs in the North Korean capital.