Seoul, June 16 : A US nuclear-powered guided missile submarine (SSGN) arrived in South Korea on Friday for the first time in six years, amid heightened tensions after North Korea's launch of two ballistic missiles, Seoul's Defence Ministry said. The Ohio-class SSGN, USS Michigan, entered a key naval base in Busan, 320 km southeast of Seoul, after North Korea fired two short-range ballistic missiles toward the East Sea on Thursday in apparent protest against the massive live-fire drills that ended earlier in the day, reports Yonhap News Agency. It marks the first time the US has sent an SSGN to South Korea since October 2017. The submarine is scheduled to remain in the country until June 21, a South Korean defence official told reportersy. The arrival of the submarine also came after the US pledged to further enhance the "regular visibility" of strategic assets on the Korean Peninsula through the Washington Declaration issued by President Yoon Suk Yeol and his American counterpart Joe Biden during their summit in April. "The US SSGN's visit to South Korea is intended to substantively implement the agreement in the Washington Declaration made in April to enhance the regular visibility of strategic assets on the Korean Peninsula," ROK Fleet Commander Vice Adm. Kim Myung-soo was quoted as saying. ROK stands for South Korea's official name, the Republic of Korea. "(It) demonstrates the overwhelming capabilities and posture of the South Korea-US alliance to realise 'peace through strength'." During the submarine's visit to the country, the South Korean and US navies plan to conduct combined special operations drills to strengthen their inter-operability and capabilities in such operations as responding to advancing North Korean threats, according to the Defence Ministry. Commissioned in 1982 as a nuclear-powered ballistic missile submarine, the USS Michigan was converted into an SSGN in 2007. The submarine is able to conduct strike missions using Tomahawk cruise missiles. The 18,000-tonne SSGN is 170 meters long and can be armed with over 150 Tomahawk missiles, which have a range of 2,500 km,. /IANS