North Korea is vowing to continue to enhance and expand its nuclear deterrent.

A spokesman for the North’s Foreign Ministry told North Korea’s state-run Korean Central News Agency that Pyongyang’s decision to develop nuclear weapons is justified, noting a recently declassified U.S. document.

According to the document, which was made public by the U.S. National Security Archive last week, Washington mulled an emergency plan to employ nuclear weapons against North Korea after the North shot down a U.S. reconnaissance plane carrying 31 crewmembers in 1969.