SEOUL, South Korea (AP) — The powerful sister of North Korean leader Kim Jong Un vowed Tuesday to boost the country’s nuclear war capability and take other steps to protest the recent arrival of a nuclear-powered U.S. submarine in South Korea.

North Korea has repeatedly vowed to expand its nuclear arsenal, but the latest threat by Kim Yo Jong came after North Korea dialed up regional tensions by unveiling a uranium-enrichment facility and testing a new ballistic missile earlier this month.

In a statement carried by state media, Kim Yo Jong said that the submarine's visit “clearly reveals the frantic military and strategic attempt of the U.S.” She said North Korea’s nuclear war deterrent must be bolstered “both in quality and quantity continuously and limitlessly” in response.

“The U.S. strategic assets will never find their resting place in the region of the Korean Peninsula,” she said. “We will continue to inform that all the ports and military bases of the ROK are not safe places." ROK stands for the Republic of Korea, South Korea’s formal name.

Her comments suggested North Korea may test-fire a missile whose range covers a South Korean site where the U.S. submarine is docked, some observers say.

South Korea’s military said the USS Vermont, a nuclear-powered and fast-attack submarine, arrived at the southeastern South Korean port city of Busan on Monday to take on supplies and allow its crew to rest.

Temporary deployments of powerful U.S. military assets like aircraft carriers, nuclear-powered submarines and bombers to South Korea are not unusual, but Washington has boosted them over the last year in a show of force against North Korea’s evolving nuclear threats.

Pyongyang often responds furiously to such visits, calling them proof of hostile intentions, and reacts with missile tests.

On Sept. 13, North Korea’s state media published photos of a secretive facility to enrich uranium for nuclear weapons. It was North Korea’s first unveiling of a uranium-enrichment facility since it showed one at the country’s main Yongbyon nuclear complex to visiting American scholars in 2010. Last week, North Korea tested a newly built ballistic missile designed to carry what it calls “a 4.5-ton super-large conventional warhead” and a modified cruise missile.

Since late May, North Korea has also floated thousands of trash-carrying balloons toward South Korea in a Cold War-style psychological campaign, prompting South Korea to restart anti-Pyongyang propaganda loudspeaker broadcasts at border areas.

South Korea’s military warned on Tuesday of unspecified military action if the North pushes its balloon campaign to a point that seriously threatens the safety of South Korean civilians.

Military spokesperson Lee Sung Joon didn’t say what action South Korea could take, but he reiterated that the military isn’t considering shooting down the balloons midair because they could be carrying hazardous substances.

So far, North Korea’s balloon activities haven't caused serious damage.

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Associated Press writer Kim Tong-hyung contributed to this report.