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Kim Yo Jong, the sister of DPRK leader Kim Jong Un, had issued a warning about the "more serious danger" that would result from a US-South Korea agreement aimed at strengthing deterrence against Pyongyang, state media reported.

Kim Yo Jong's remarks came after US President Joe Biden and his South Korean counterpart threatened to end DPRK's leadership if it uses its nuclear arsenal, issuing what was called the Washington Declaration in which the US will pledge to bolster its commitment to a so-called "extended deterrence" to counter the threats posed by the DPRK and mobilize a wide range of its military capabilities.

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Earlier this week, South Korean President Yoon Suk Yeol visited Washington to plead for security assurances from US President Joe Biden.

In Pyongyang's first response to the Washington summit, Kim Yo Jong said DPRK remained convinced that its nuclear deterrent "should be brought to further perfection."

"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," she said, as per the official Korean Central News Agency (KCNA).

The agreement will involve the "regular deployment of strategic assets," including the first South Korean port visit by a nuclear ballistic submarine in decades, a US official said as quoted by AFP.

The agreement, however, would "only result in making peace and security of Northeast Asia and the world be exposed to more serious danger, and it is an act that can thus never be welcome," Kim Yo Jong affirmed.

This is happening as Washington and Seoul stepped up drills simulating a landing in the DPRK, further stoking tensions in the peninsula.

'Old man with no future'

Elsewhere in her remarks, Kim Yo Jong lashed out at US President Joe Biden, stating that the latter's criticism of the DPRK is "nonsensical...from the person in his dotage."  

She said Saturday that he was "not at all capable of taking the responsibility... an old man with no future," referring to 80-year-old Biden's age.

She added it was "too much for him to serve out the two-year remainder of his office term."

On its account, Seoul condemned Kim Yo Jong's "far-fetched" statement, saying it "reflects its nervousness and frustration at the drastically strengthened nuclear deterrence of the Korea-US alliance," as per South Korea's Unification Ministry.

As South Korea weaves closer ties with the US, DPRK's leader declared last year that his country is an "irreversible" nuclear power, conducting weapon tests nearly every month, including the launch of its most advanced intercontinental ballistic missile (ICBM).

Read next: US-South Korea drills driving tensions to 'brink of nuclear war': DPRK