SEOUL, South Korea (AP) — North Korea lashed out Friday at the arrival of a U.S. aircraft carrier battle group in South Korea, calling it a provocation and again raising the specter of using nuclear weapons to defend itself.

Emboldened by its advancing nuclear arsenal, North Korea has increasingly issued threats to use such weapons preemptively. But the North is still outgunned by U.S. and South Korean forces, and experts say it is unlikely to use its nukes first, though it will continue to upgrade those arms without returning to diplomacy for the time being.

The North’s latest nuclear threat came a day after the USS Ronald Reagan and its battle group arrived at South Korea’s southeastern port of Busan, following U.S.-South Korean-Japanese naval exercise in international waters earlier this week.

South Korean defense officials said the carrier is to be docked at Busan for five days as part of an agreement to increase the temporary deployments of powerful U.S. military assets in response to the North’s growing nuclear program.

On Friday, the North’s official Korean Central News Agency called the aircraft carrier’s arrival “an undisguised military provocation” that proves a U.S. plan to attack North Korea is being realized. It threatened to respond in line with its escalatory nuclear doctrine that authorizes the preemptive use of nuclear weapons

“The (North Korean) doctrine on the use of nuclear weapons already opened to public allows the execution of necessary action procedures in case a nuclear attack is launched against it or it is judged that the use of nuclear weapons against it is imminent,” the KCNA dispatch said.

North Korea’s “most powerful and rapid first strike will be given to the ‘extended deterrence’ means, used by the U.S. to hallucinate its followers, and the bases of evil in the Korean peninsula and its vicinity,” KCNA added.

North Korea has argued it was forced to develop nuclear weapons to cope with what it calls the U.S. and South Korean plots to invade. It has often made furious responses to the deployment of U.S. strategic assets like aircraft carriers, long-range bombers and nuclear-powered submarines as well as U.S. joint training exercises with South Korean forces.

Many experts say North Korea heightens tensions with its rivals to provide a pretext for expanding its nuclear arsenal and then uses the arms as leverage to wrest greater outside concessions.

Since last year, North Korea has conducted more than 100 missile tests in the name of responding to the expanded U.S.-South Korean military drills. Washington and Seoul say their drills are defensive in nature.

Last year, North Korea adopted a law that stipulates a broad range of situations in which it can use nuclear weapons, including when it determines that its leadership faces imminent attack by hostile forces or when it needs to prevent an unspecified catastrophic crisis to its people and government.

The U.S. and South Korean governments have repeatedly warned that any attempt by North Korea to use nuclear weapons would result in the end of the North’s government led by Kim Jong Un.