North Korea has threatened South Korea that it will it will attack its enemy with "a merciless retaliation without warning" via a fax message, according to a report by the Wall Street Journal.

Kim Min-seok, spokesperson of South Korea's Defense Ministry, confirmed the report and said that a faxed letter came from the North's National Defense Commission addressed to the South's presidential office, last Thursday, December 19.

The faxed message came right after demonstrations in the South were held by conservative activist and North Korean refugee groups to mark the second death anniversary of Kim Jong Il this week, says the WSJ report.

The protest was shown on national television with protesters burning the effigies of three North Korean dictators.

The North's response to the protest was the faxed message with a promise of "resolute punishment", the report says.

This is not the first time the North has used this tactics - sending faxed messages - to the South. Earlier this year, South Korean firms that run factories in Kaesong Industrial Complex, a jointly-run complex inside North Korea, also received faxed messages blaming Seoul for the plants' prolonged closure, the WSJ says.

The two Koreas also have other modes of communication besides faxes. One is the daily phone calls that take place at the border of the two countries to coordinate the traffic of people into the Kaesong complex. According to a spokeswoman at Seoul's Ministry of Unification, civilian officials talk to each by exchanging physical documents other via an administrative office at the Kaesong plant.

However, there is no email communication between the two countries.

But there is a more unconventional approach used by South Korean activists called leaflet flights. They send information condemning North Korea's regime using helium-filled balloons northward.

The North has also sent leaflets to the South although no one is sure how they are delivered. This week, local media reports that leaflets were found on a South Korean border island with threats coming from the North.