The warning came Sunday as naval forces of the US and South Korea would be conducting a joint drill Monday and Tuesday with Japan in southern waters off the Jeju Island, Xinhua reported.
"If they persist in doing so, all the self-defensive deterrent forces of North Korea will keep themselves fully ready to punish all undesirable forces," North Korea's National Defence Commission (NDC) said in a statement.
According to the statement, North Korea has put in efforts towards mending the relations and achieving a reunification with South Korea.
It also blamed the UN Security Council for losing its impartiality and misleading public opinion.
On July 16-20, the US and South Korea conducted joint naval exercises that involved aircraft carrier USS George Washington and South Korean battleships in the southern waters off the Korean Peninsula.