SEOUL, South Korea (NEWSnet/AP) — The United States deployed a nuclear-armed submarine to South Korea on Tuesday for the first time in four decades, as the allies warned North Korea that use of of nuclear weapons in combat would result in the end of its regime.

Periodic visits by U.S. nuclear ballistic missile-capable submarines to South Korea were one of several agreements reached by the two countries’ presidents in April, in response to North Korea’s expanding nuclear threat. They also agreed to establish a bilateral Nuclear Consultative Group and expand military exercises.

USS Kentucky, an Ohio-class submarine, arrived at the South Korean port of Busan on Tuesday, according to South Korea’s Defense Ministry. It is the first visit by a U.S. nuclear-armed submarine to South Korea since the 1980s, it said.

Defense Minister Lee Jong-Sup called the submarine’s visit a demonstration of U.S. resolve in implementing its “extended deterrence” commitment, a pledge by the U.S. to use its full military capabilities, including nuclear weapons, to protect its allies, the ministry said.

North Korea’s nuclear ambitions have taken on a new urgency after it threatened to use nuclear weapons in conflicts with its rivals and conducted about 100 missile tests since the start of last year. Last week, North Korea conducted a second test of a more mobile and powerful intercontinental ballistic missiles designed to strike the mainland United States. After observing that launch, North Korea leader Kim Jong Un vowed to strengthen his country’s nuclear combat capabilities.

During the Cold War in the late 1970s, U.S. nuclear-armed ballistic missile submarines made frequent visits to South Korea, sometimes two or three times per month, according to the Federation of American Scientists.

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