SEOUL, South Korea — North Korea threatened Saturday to boost its nuclear fighting capability and make the U.S. and South Korea pay 'an unimaginably harsh price' as it slammed its rivals' new defense guidelines that it says reveal an intention to invade the North.

On Thursday, President Joe Biden and South Korean President Yoon Suk Yeol authorized the signing of joint nuclear deterrence guidelines as part of efforts to enhance their capabilities to cope with North Korea's growing nuclear arsenal. The guidelines were adopted a year after the two countries established a consultation body to bolster information-sharing on nuclear operations and discuss how to integrate U.S. nuclear weapons and South Korean conventional weapons in contingencies.