North Korea threatened on Saturday to make the United States and South Korea pay "an unimaginably harsh price," criticizing the two nations for signing their new defense guidelines to respond to North Korean nuclear attacks.

In a statement carried by the North's official Korean Central News Agency (KCNA) on Saturday, North Korea's defense ministry spokesperson said the North strongly denounces and rejects the reckless provocative acts of the United States and South Korea that escalate regional military tensions through extremely incendiary and malicious rhetoric and provocative action.

The spokesperson said their provocative acts urgently require North Korea to further improve its nuclear deterrent readiness and add unspecified "important elements to the composition of the deterrent."

The ministry then warned that the United States and South Korea will "pay an unimaginably harsh price" if they fail to stop provocative acts.

The statement came after Seoul and Washington signed the joint guidelines on Thursday as part of efforts to enhance their capabilities to cope with North Korea's growing nuclear arsenal.