The South Korean military on Sunday evacuated gunboats in the West Sea after detecting signs that North Korea activated the radar for surface-to-ship missiles deployed on the Ongjin Peninsula in Hwanghae Province.

"Around 1 p.m., we detected signs of the North activating radar for surface-to-ship missiles and coast artillery batteries deployed at Cape Jangsan and on the coast south of the cape," a military source said. "We evacuated 1,200 ton-class naval patrol boats and speedboats near Daecheong and Yeonpyeong islands from the range of the missiles and coast artillery pieces to safety."

Radar for missiles or coast artillery is usually activated to launch an attack or carry out a live firing exercise. It was not known why the North activated it.

An officer with the Joint Chiefs of Staff said, "The North has activated radar for surface-to-ship missiles and coast artillery during exercises in the past, but we evacuated the ships to safety as we feared that the North might seek to retaliate" after a skirmish in the West Sea on Nov. 10.

North Korean surface-to-ship Silkworm missiles (top) and 130 mm coast artillery

The chief of the North Korean military delegation to inter-Korean generals' talks sent a message to his South Korean counterpart on Friday warning of "military measures" in the wake of the skirmish. "There exists only the maritime military demarcation line drawn by us in the West Sea. From this moment on, we are going to take merciless military measures to defend it," the message said, referring to a unilaterally drawn sea border south of the official Northern Limit Line.

The North Korean military radar was activated for about an hour, and there were no signs of it being activated again, the source added. The South Korean military has been on alert for a retaliatory attack or an armed provocation in the demilitarized zone.

Between January and May, when it ratcheted up tensions, North Korea carried out a drill firing about 1,000 artillery shells near Yeonpyeong Island.

South Korea has deployed near the NLL two 4,500 ton-class destroyers, both equipped with the Korean-made ship-to-ship cruise missile Haeseong with a range of 150 km, patrol boats, and a 1,800 ton-class convoy ship. The Air Force has also reportedly intensified patrol and reconnaissance activities over waters near Incheon and Yeongjong Island.