North Korean leader Kim Jong-un`s powerful sister has warned that a recent US-South Korean deal will lead to a "more serious danger".
Kim Yo-jong said North Korea`s nuclear deterrent "should be brought to further perfection", state media reported.
The US agreed to deploy nuclear-armed submarines to South Korea and involve Seoul in its nuclear planning to counter nuclear threats from the North.
In return, South Korea agreed to not develop its own nuclear weapons.
The deal - known as the Washington Declaration - was announced this week during President Joe Biden`s talks with his South Korean counterpart Yoon Suk-yeol`s in the American capital .
Referring to the agreement, Ms Kim said: "The more the enemies are dead set on staging nuclear war exercises, and the more nuclear assets they deploy in the vicinity of the Korean peninsula, the stronger the exercise of our right to self-defence will become in direct proportion to them."
She warned that the move would "only result in making peace and security of North-East Asia and the world be exposed to more serious danger".
Ms Kim holds a senior position in the Central Committee of the Workers` Party of Korea, and is reported to have influence on her brother.
President Biden hailed the deal, saying it would strengthen the allies` co-operation in deterring a North Korean attack.
President Yoon said the agreement marked an "unprecedented" commitment by the US to deter attacks and protect allies by using nuclear weapons.
China warned against "deliberately stirring up tensions, provoking confrontation and playing up threats".
Under the deal, the US will make its defence commitments more visible by sending a nuclear-armed submarine to South Korea for the first time in 40 years, along with other assets, including nuclear-capable bombers.
The two sides will also develop a Nuclear Consultative Group to discuss planning issues.
Politicians in Seoul have long been pushing Washington to involve them more in planning for how and when to use nuclear weapons against North Korea.
Concern has been rising on both sides about the nuclear threat posed by North Korea. Pyongyang is developing tactical nuclear weapons that can target South Korea, and refining its long-range weapons that can reach the US mainland.
The US already has a treaty obligation to defend South Korea, and has previously pledged to use nuclear weapons if necessary. But some in South Korea have started to doubt that commitment and call for the country to pursue its own nuclear programme.
