Seoul: Seoul's military reported that a US Navy nuclear-powered submarine arrived in the port city of Busan, South Korea, on Friday, demonstrating Washington's commitment to counter Pyongyang's increasing threats.

The North Korean leader Kim Jong Un has declared his country to be a "irreversible" nuclear state and has called for increased weapon development, including tactical nukes, as relations between the two Koreas are at one of their lowest points. Diplomacy has stalled.

Seoul and Washington retaliated by threatening Pyongyang with nuclear retaliation and the "end" of the current regime in North Korea should it ever use its own nuclear weapons against the allies.

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

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

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

The Joint Chiefs of Staff (JCS) released a statement in which they stated that Seoul and Washington intended to "strengthen their special warfare capabilities and interoperability to respond to North Korea's growing threats through joint special warfare drills," in conjunction with the submarine port call in Busan.

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According to reports, the 18,000-ton submarine is about 170 metres long and has the capacity to carry 150 Tomahawk missiles with a 2,500-kilometer range.

This year, North Korea has conducted a number of launches that violated sanctions, including test-firing some of its most potent intercontinental ballistic missiles and attempting to launch a military spy satellite last month.

On Thursday, Pyongyang launched two ballistic missiles in apparent retaliation to ongoing joint military exercises between the US and South Korea. North Korea views all such drills as invasion practise and has said that they are "frantic" exercises "simulating an all-out war against" Pyongyang.