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

In response, Seoul and Washington vowed that Pyongyang would face a nuclear response and the "end" of the current government in North Korea, were it to ever use its own nukes against the allies.

The USS Michigan, an Ohio-class nuclear-powered guided missile submarine, arrived in Busan on Friday for the first time in six years, the South's military said, in accordance with the Washington declaration signed by US and South Korean leaders in April.

The declaration states Washington's "commitment to extend deterrence to South Korea is backed by the full range of US capabilities, including nuclear", to counter Pyongyang's growing threats, South Korea's Fleet Commander Kim Myung-soo said.

The submarine's arrival is "intended to substantively implement the Washington Declaration... to enhance the regular visibility of US strategic assets on the Korean Peninsula", he said.

With the submarine port call in Busan, Seoul and Washington plan to "strengthen their special warfare capabilities and interoperability to respond to North Korea's growing threats through joint special warfare drills," the Joint Chiefs of Staff (JCS) said in a statement.

The 18,000-ton submarine is around 170 metres (560 feet) long and can be equipped with 150 Tomahawk missiles with a range of 2,500 kilometres (1,553 miles), according to the JCS.

North Korea has conducted multiple sanctions-busting launches this year, including test-firing its most powerful intercontinental ballistic missiles, and last month attempting to put a military spy satellite into orbit.

Pyongyang on Thursday fired two ballistic missiles in an apparent response to ongoing US-South Korea joint military drills.

North Korea regards all such exercises as rehearsals for invasion and has described them as "frantic" drills "simulating an all-out war against" Pyongyang.