North Korean leader Kim Jong Un's powerful sister warned Saturday that a U.S.-South Korean agreement aimed at strengthening deterrence against Pyongyang will lead to "more serious danger," state media reported.

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

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