South Korean military vessels have begun an operation to repel Chinese fishing boats illegally harvesting prized blue crabs from an area near Seoul's disputed sea boundary with North Korea.

South Korea's Defense Ministry said Friday that four naval and marine boats are in neutral waters around Ganghwa island for an operation approved by the United Nations Command that governs the zone.

The operation comes days after South Korean fishermen towed away two Chinese fishing boats catching crabs south of the sea boundary with North Korea and handed them over to local South Korean authorities. North Korea said in response that South Korean fishing and naval vessels had invaded their territory.

Seoul has called for Beijing to employ tougher measures against Chinese boats illegally fishing in South Korea-controlled waters.