Kim Yo Jong, the powerful sister of North Korea's leader Kim Jong Un, has warned that the new US-South Korea agreement would only lead to "more serious danger," reported the state media on Saturday.

South Korean President Yoon Suk Yeol was in Washington earlier this week to seek security assurances from US President Joe Biden.

After the summit on Wednesday, Biden said that any North Korean nuclear attack on the US or its allies would "result in the end of whatever regime" took such action.

Kim Yo Jong said that the summit only strengthened North Korea's resolve to enhance its nuclear arms capabilities and that a nuclear deterrent "should be brought to further perfection."

North Korea: Biden should prepare for an 'after-storm'

A key element of the renewed US expression of support for Seoul is the promise to have US nuclear submarines make port calls in South Korea, something that has not taken place for over 40 years.

The two countries also committed to plans for bilateral presidential consultations in the event of a nuclear attack from North Korea, the establishment of a nuclear consultative group and improved information exchange.

Kim, who is her brother's top foreign policy official, stated that the agreement reflected the "most hostile and aggressive will of action" against North Korea and threatens regional peace and security, reported KCNA, the state media agency.

She also lashed out at Biden, calling him senile and "too miscalculating and irresponsibly brave."

However, she added that Pyongyang wouldn't simply dismiss his words and that he "should be prepared for far too great an after-storm."

A looming threat, amid heightened tensions

In recent months, North Korea's weapons demonstration and the US-South Korea military drills have increased in a tit-for-tat cycle.

Biden's meeting with Yoon came amid this heightened tension in the Korean Peninsula.

In a record-breaking, sanction-defying move, North Korea test-fired around 100 missiles since the start of 2022. Kim Jong Un is widely expected to up the ante in the coming months as he continues to assert the country's nuclear prowess.

South Korea has also expanded its military training with the US and called nuclear scientists to increase production of weapons-grade material.