USS Michigan arrives for a regularly scheduled port visit in Busan, South Korea,
© Jermaine Ralliford / Reuters The Ohio-class guided-missile submarine USS Michigan arrives for a regularly scheduled port visit while conducting routine patrols throughout the Western Pacific in Busan, South Korea, April 24, 2017.
A US nuclear submarine armed with guided missiles has made a port call in South Korea, while Pyongyang is reportedly holding massive long-range artillery drills amid muscle-flexing in the region and rising tensions between Washington and North Korea.

The USS Michigan nuclear submarine arrived in South Korea on Tuesday "during a regularly scheduled deployment to the Western Pacific," according to a statement issued by the US Pacific Fleet.

The submarine is one of the four Ohio-class nuclear vessels, one of the largest in the world "measuring more than 560 feet long and weighing more than 18,000 tons when submerged."

The US military praises the sub as a weapon to "provide the Navy with unprecedented strike and special operation mission capabilities."

The move comes at a tense time in relations between Washington and Seoul on one side and North Korea on the other.

On Tuesday, Pyongyang celebrates the 85th anniversary of the foundation of its Korean People's Army. The jubilee was marked by massive - yet conventional - drills, which featured some 300-400 long-range artillery pieces and were supervised by North Korean leader Kin Jong-un, according to Yonhap News agency citing South Korean Army sources.