North Korea is known to be preparing for a large-scale military exercise on the Yellow Sea.

A South Korean military source says vessels are mobilized at a Navy base in Nampo in North Korea's South Pyeongan Province and fighter jets are gathered at an Air Force base in Oncheon in the same province.

The source says the preparations of the military drill had been detected, and although the weather will be a factor, the drills are expected to take place after Wednesday, marking the 58th anniversary of the Korean War armistice.

Some 20 landing vessels, warships and LCACs, or landing craft air-cushion vehicles, are believed to be on stand-by near Nampo. MiG jet fighters have also been deployed from a base in Wonsan, Gangwon Province to Oncheon, South Pyeongan Province.

The source says the North Korean military does conduct summer drills, but a Navy and Air Force joint exercise is unusual around the anniversary of the armistice.

The source says the move could be in response to South Korea's establishment of a defense command last month that is aimed at better defending the nation's northwestern frontline islands.