SEOUL, July 21 (Yonhap) -- A U.S. strategic nuclear ballistic missile submarine (SSBN) left South Korea on Friday, an informed source said, three days after its rare port visit to the southeastern city of Busan in a major show of force against evolving North Korean military threats.

The USS Kentucky, an 18,750-ton Ohio-class submarine, departed from a naval base in the city, 320 kilometers southeast of Seoul, before noon, according to the source who requested anonymity. It remains unknown where it is headed.

The SSBN arrived at the base Tuesday, in what was seen as a move to reassure the South Korean public, which is increasingly wary of North Korean nuclear and missile threats, and deter those threats.

The first U.S. SSBN visit to South Korea since 1981 came after the United States pledged to enhance the "regular visibility" of its key military assets, including the submarine, in the Washington Declaration that Presidents Yoon Suk Yeol and Joe Biden issued during their summit in April.

The SSBN's presence drew the ire of North Korea. Pyongyang's Defense Minister Kang Sun-nam warned Thursday that the deployment of the nuclear submarine and other strategic assets to the South may fall under the conditions for the North's use of nuclear arms.

The USS Kentucky is among 14 Ohio-class SSBNs and is homeported at Naval Base Kitsap in the state of Washington. It is capable of carrying some 20 Trident-II D5 ballistic missiles that have a range of 12,000 km.