SEOUL, April 29 (AFP) - North Korean leader Kim Jong Un's powerful sister Kim Yo Jong has warned that a US-South Korean agreement aimed at strengthening deterrence against Pyongyang would only lead to "more serious danger," state media reported Saturday.

The United States and South Korea vowed Wednesday that North Korea would face a nuclear response and the "end" of the leadership there should it use its own arsenal, as the two countries' presidents met in Washington.

In response, Kim Yo Jong said the North was convinced that a 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-defence will become in direct proportion to them," she said, according to the official Korean Central News Agency.

South Korean President Yoon Suk Yeol and US counterpart Joe Biden issued what was titled the Washington Declaration, bolstering the US nuclear umbrella over South Korea, which is increasingly nervous about Pyongyang's aggression.

It 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 told AFP this week.

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 said.

North Korea has defied years of punishing sanctions to continue work on its banned nuclear and missile programmes, and has indicated it will not consider giving up weapons it views as insurance against regime change.

Pyongyang conducted a record-breaking string of sanctions-defying launches this year, including test-firing the country's first solid-fuel ballistic missile -- a key technical breakthrough for Kim Jong Un's military.