North Korea has expanded its own loudspeaker broadcasts along the inter-Korean border as a counteraction to South Korea's retaliatory broadcasts critical of the communist nation, sources said Monday.

In retaliation for North Korea's nuclear test last Wednesday, the South resumed its anti-Pyongyang broadcast campaign two days later, a form of psychological warfare detested by the communist country, where outside information is tightly blocked out. 

"The North initially operated its own loudspeakers at two locations and has now expanded to several locations," a government source said. "In fact, the anti-South loudspeaker broadcasts appear to be coming from every location where we are broadcasting."

South Korea is currently airing the propaganda broadcasts at 11 points along the tensely guarded border.

The North Korean broadcasts are not clearly audible from the South Korean side of the border, but mostly deal with internal propaganda messages and music promoting its leader Kim Jong-un, according to the sources.

The messages also contain criticisms of President Park Geun-hye, they noted. 

"We are not sure if it's an issue of electric power or the performance of the loudspeakers, but the sound is very weak," another government source said. 

The South Korean loudspeaker campaign marks its fourth day on Monday, with North Korea showing no signs of military action over the broadcasts.

The South Korean military put its alert level at the broadcasting area to the most stringent stage for the possibility of a North Korean military action. (Yonhap)