North Korea threatened Saturday to boost its nuclear fighting capability and make the US and South Korea pay “an unimaginably harsh price” as it slammed its rivals’ new defence guidelines that it says reveal an intention to invade the North.

On Thursday, President Joe Biden and South Korean President Yoon Suk Yeol authorised 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 for information-sharing on nuclear operations.