Warning comes days after the US sent nuclear submarine to South Korea for the first time in decades.

Kang Sun Nam’s comments, reported on Thursday by the Korean Central News Agency (KCNA), came in response to the US sending a submarine with nuclear-armed ballistic missiles to South Korea this week for the first time in decades.

“The ever-increasing visibility of the deployment of the strategic nuclear submarine and other strategic assets may fall under the conditions of the use of nuclear weapons specified in the DPRK law,” Kang said in a statement, referring to his country by its official name, the Democratic People’s Republic of Korea.

Tensions between the US and North Korea have been on the rise in recent weeks with Pyongyang increasing its ballistic missile testing in defiance of Washington and international sanctions.

Late last week, the US, South Korea and Japan released a joint statement condemning the launch of a North Korean intercontinental ballistic missile days earlier.

“The United States reiterated that its commitments to defend the ROK [Republic of Korea, or South Korea] and Japan are ironclad and backed by the full range of capabilities, including nuclear,” the statement said.

The three allies went on to hold a joint military drill on Sunday.

On Wednesday, South Korea and Japan reported that North Korea had launched two more ballistic missiles.

The US and South Korea also held their first so-called Nuclear Consultative Group meeting this week. The White House said it provided a chance for the US to reaffirm its commitment to provide “extended deterrence” to South Korea.

“Any nuclear attack by North Korea against the United States or its allies is unacceptable and will result in the end of that regime, and the US and ROK sides highlighted that any nuclear attack by the DPRK against the ROK will be met with a swift, overwhelming, and decisive response,” the White House said in a statement after the talks.

On Thursday, North Korea slammed the nuclear meeting. “The phase of a military clash on the Korean Peninsula has surfaced as a dangerous reality,” the KCNA report said.

US soldier crosses border into North Korea

Amid the escalation, 23-year-old US soldier Travis King intentionally crossed into North Korea this week. US officials believe he is now in North Korean custody.

King had served nearly two months in a South Korean prison for assault and was due to travel back to the US, but he skipped his flight, joined a tour group to the demilitarised zone and subsequently ran across the border into North Korea at the village of Panmunjom.

On Wednesday, the US Department of State said the Pentagon had reached out to the North Korean military about the case but “those communications have not yet been answered”.

Former US President Donald Trump engaged in direct talks with North Korean leader Kim Jong Un during his tenure, but high-level meetings between the two countries came to a halt under the current US president, Joe Biden.

After the first meeting between Trump and Kim in 2018, the nations said in a joint statement that North Korea was committed to “work toward complete denuclearization of the Korean Peninsula”.

But the pledge was never followed by efforts to end the country’s nuclear weapons programme.

North Korea carried out its first nuclear weapons test in 2006 in violation of an international ban on such testing. Since then, the United Nations Security Council has unanimously passed numerous resolutions that have imposed sanctions on the country over its nuclear programme.

Last year, Russia and China vetoed a Security Council proposal to impose more penalties on North Korea, arguing that sanctions have not been effective in curbing its nuclear and missile programmes.