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

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

On Tuesday, they held the first Nuclear Consultative Group (NCG) meeting in Seoul, which aims to improve nuclear coordination between the two allies and boost military readiness against North Korea.

"As we speak, an American nuclear submarine is making port in Busan today, the first visit of American nuclear submarine in decades," White House Indo-Pacific coordinator Kurt Campbell told reporters after the meeting.

The last time Washington deployed one of its nuclear-capable submarines to South Korea was in 1981.

Washington announced it would deploy a submarine capable of launching ballistic missiles with nuclear warheads to the Korean peninsula in April, while South Korean President Yoon Suk Yeol was on a state visit. It did not specify a timeline.

"The US side demonstrated strong resolve that in case the North attacks the South with nuclear weapons it will be met with immediate, overwhelming and decisive counter measures, leading to the demise of its regime," Kim Tae-hyo, the national security adviser who co-chaired the NCG meeting with Campbell, told reporters.

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

Kim Yo Jong, the powerful sister of leader Kim Jong Un, said Monday that such actions would only "make the (Democratic People's Republic of Korea) go farther away" from possible talks.

North Korea was "ready for resolutely countering any acts of violating its sovereignty", said Kim Yo Jong, who also dismissed holding talks with the United States as a "daydream".