North Korea says the arrival at South Korea by a US nuclear-capable submarine this week may meet legal conditions for Pyongyang to use nuclear weapons.

North Korea's defense minister Kang Sun Nam issued the warning in a statement reported by the state-run Korean Central News Agency on Thursday.

Kang says the US scenario for a nuclear attack on North Korea and its implementation "have entered the most critical stage of visualization and systemization."

He also says that "the phase of a military clash on the Korean Peninsula has surfaced as a dangerous reality."

The defense minister adds that "the ever-increasing visibility of the deployment" of strategic assets may fall under the conditions for the use of nuclear weapons specified in North Korea's law.

The submarine, the USS Kentucky, is capable of carrying ballistic missiles equipped with nuclear warheads. It made a port call in South Korea on Tuesday, becoming the first such vessel to do so in 42 years.

South Korean President Yoon Suk-yeol boarded and inspected the sub on Wednesday.