Kim Yo-jong, the influential sister of North Korean leader Kim Jong-un, vowed Tuesday to "continuously and limitlessly" bolster the North's nuclear war deterrent against what it called US threats, denouncing the arrival of a US nuclear-powered submarine in South Korea.

The remark came a day after the 7,800-ton USS Vermont entered a major naval base in Busan, 320 kilometers southeast of Seoul, to replenish supplies and provide rest for crew members.

"The DPRK's nuclear war deterrent to cope with and contain various threats from outside is bound to be bolstered up both in quality and quantity continuously and limitlessly as the security of the state is constantly exposed to the US nuclear threat and blackmail," Kim said in a statement carried by the North's Korean Central News Agency.

Kim claimed that the visit is "proof that the US ambition to often take up nuclear strategic assets, boast of its strength, increase its threat to the rival and 'enjoy' its hegemonic privilege by dint of its malicious strength at any cost is getting ever more extreme."

She still said "nothing will change," stressing that such nuclear-powered submarines can never be an "object of fear."

"The US strategic assets will never find their resting place in the region of the Korean peninsula," Kim said.

In what appeared to be an attempt to boast about the North's surveillance capabilities, Kim said the North's Aerospace Reconnaissance Agency, an independent intelligence organ directly under the head of state, detected the submarine's arrival and reported it.

The KCNA did not carry photos supporting the surveillance report.

In November last year, Pyongyang successfully placed a spy satellite into orbit, and said it has a plan to launch three more such satellites in 2024. Its attempt to launch another satellite in May ended in failure.

Earlier this year, South Korea's National Security Advisor Shin Won-sik, who was then defense minister, told reporters that the North's Malligyong-1 spy satellite appears to be orbiting Earth without activity. (Yonhap)