North Korea has warned of more provocative military displays in retaliation to President Biden's remarks that an attack on the US or its allies would lead to the "end of the regime".
Reacting to the news the US and South Korea have formed a pact to counter the threat from Pyongyang, Kim Yo Jong – the sister of leader Kim Jong Un – said the agreement reflected a "hostile and aggressive will of action" against the North.
And the pact will put peace in the region in "more serious danger," she said.
Mr Biden met his South Korean counterpart, Yoon Suk Yeol, in Washington this week.
He said a nuclear attack would "result in the end of whatever regime".
The summit saw Mr Biden and Mr Yoon unveil new nuclear deterrence efforts calling for US nuclear-armed submarines to dock periodically in South Korea for the first time in decades, together with increased training between the two countries.
They have also committed to plans for bilateral presidential consultations if North Korea does launch a nuclear attack.
A nuclear consultative group and improved sharing of information on nuclear and strategic weapons operation plans are also part of the pact.
Biden should be prepared for 'storm'
Kim Yo Jong said the summit served to further strengthen Pyongyang's determination to enhance its nuclear arms capabilities.
She lashed out at Mr Biden for being "too miscalculating and irresponsibly brave" – and branded his comments "nonsensical remarks from the person in his dotage".
"When we consider that this expression was personally used by the president of the US, our most hostile adversary, it is threatening rhetoric for which he should be prepared for far too great an after storm," she warned.
She added: "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."
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North Korea earlier warned Washington and Seoul that a move to shoot down one of its test missiles would be considered a "declaration of war".
In March, Kim Yo Jong blamed a joint military exercise between the US and South Korea for growing tensions on the peninsula, according to state media.