SEOUL (CBS DC) – Tensions between North and South Korea kicked up a notch after a threatening fax was sent from Pyongyang to Seoul on a special communications line between the two rival capitals.

In it, the North’s National Defense Commission warned it could strike “without notice,” reports South Korea’s Yonhap News Agency.

The move came as conservative groups and North Korean defectors held rallies against Kim Jong-Un regime.

The protests were timed to coincide with the second anniversary of the death of his father, Kim Jong-Il.

The North Korean military condemned those rallies as an insult to the country’s “highest dignity.”

Kim Jong-Il recently ordered the execution of his uncle Jang Song-Thaek in order to consolidate his grip on power.

After the execution, South Korean President Park Geun-Hye ordered military leaders to revise plans to cope with the changing situation on the peninsula.

South Korea responded to the threat by sending its own message to Pyongyang vowing to “sternly react” to any provocation from the North.