The powerful sister of North Korean leader Kim Jong-un slammed a new South Korea-US agreement bolstering Washington's nuclear deterrence efforts against North Korean threats on Saturday, saying the allies' plan will only result in "more serious danger."

In North Korea's first response to the Washington Declaration adopted during President Yoon Suk Yeol's state visit to the US, Kim Yo-jong said that the agreement reflects "the most hostile and aggressive will of action" against the North that will "only result in making peace and security of Northeast Asia and the world be exposed to more serious danger," according to the Korean Central News Agency (KCNA).

Kim's remarks came three days after Yoon and US President Joe Biden agreed to significantly strengthen Washington's nuclear commitment to South Korea in the new agreement and promised to take "swift, overwhelming, and decisive" action in the event of North Korea's nuclear attack.

Under the Washington Declaration, Yoon and Biden agreed to strengthen the US extended deterrence commitment to South Korea through the establishment of a Nuclear Consultative Group and more frequent deployments of U.S. strategic assets to South Korea.

Kim, who is known for her influence on inter-Korean affairs in the Kim regime, holding the post of vice department director of the Central Committee of the Workers' Party of Korea, warned that the change in security environment will only push the North to take "more decisive action."

"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-defense will become in direct proportion to them," she was quoted saying in the English-language dispatch.

Kim also lashed out at both leaders, lambasting Biden's warning that any nuclear attack against the US or its allies will result in the end of its regime.

Saying that Biden was "too miscalculating and irresponsibly brave," Kim painted the warning as "nonsensical remark from the person in his dotage."

"It may be taken as a nonsensical remark from the person in his dotage who is not at all capable of taking the responsibility for security and the future of the US, an old man with no future, as it is too much for him to serve out two-year remainder of his office term," Kim said.

She also took a swipe at Yoon, calling him a "fool" who has "put the security into crisis with his incompetence."

"The pipe dream of the US and South Korea will henceforth be faced with the entity of more powerful strength," Kim said.

Meanwhile, Seoul's unification ministry in charge of inter-Korean affairs condemned Kim's remarks, calling them "far-fetched" claims.

"North Korea hastily issuing far-fetched claims right after the announcement of the Washington Declaration appears to reflect its nervousness and frustration at the drastically strengthening nuclear deterrence of the South Korea-U.S. alliance," the ministry said in a statement.

It warned that North Korea would face "even stronger and overwhelming" response if it continues to follow the "wrong path," while saying Kim's slandering of the two countries' leaders in "rude language" demonstrated the regime's "lowly level."

The KCNA report came as Yoon's state visit to the US is under way. (Yonhap)