North Korea on Friday threatened to attack South Korea without any notice via a fax sent to South Korea’s National Security Council, the Ministry of Defence said in Seoul.

The fax made reference to recent demonstrations in which effigies of Kim Jong Un were burnt in Seoul on the anniversary of Kim Jong Il’s death, an issue that often aggravates the North.

South Korea reacted firmly to Pyongyang’s warning that it would “mercilessly” attack “without notice” by sending a fax back that promised “resolute punishment” to any attack initiated by the North.

Later, South Korea’s unification minister denounced the attack, calling