North Korea is preparing for a military exercise on the west coast involving all three branches of its forces, a government source here said Tuesday.

"We noted preparations for a military exercise from the North's naval base in Nampo and its aerial base in Onchon in South Pyongan Province," the source said, referring to a western province north of the border. "Warships and fighters have been assembled there. It depends on the weather, but we believe they're setting up for a landing exercise sometime after Wednesday."

That day is the 58th anniversary of the signing of the armistice that ended the 1950-53 Korean War. The truce has left the two Koreas technically at war with each other.

"North Korean forces do carry out summer exercises, but it'd be unusual for them to have one around the armistice anniversary," the source said. "We believe this may be in response to our opening of a new command last month to better protect islands near the western sea border."

South Korea launched the Northwest Islands Defense Command in June, following North Korea's shelling of Yeonpyeong Island near the border last November that killed four people.

The Yellow Sea border has been a constant source of military tensions between the two Koreas, with bloody naval skirmishes taking place there in 1999, 2002 and 2009. (Yonhap News)