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

Four naval and marine boats were in neutral waters around South Korea's Ganghwa island to chase away about 10 Chinese boats.

The operation was approved by the United Nations Command that governs the zone where fishing activity is prohibited, said an official from Seoul's Defense Ministry, who didn't want to be named, citing office rules.

"United Nations Command takes its responsibility to maintain the armistice very seriously. We had a responsibility to act and we are doing that," Gen. Vincent Brooks, the U.S. commander of the United Nations Command, said in a statement on the decision to authorize the operation.

The governments of China and North Korea were notified about the operation before it started, the Seoul ministry official said. The official refused to say how the vessels were planning to repel the ships, or provide further details about the operation.

The operation came 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, which has caused bad feelings between the neighbors in recent years.

South Korean authorities seized about 600 Chinese ships last year for illegal fishing and more than 100 this year as of May, most from waters off the western coast of South Korea, according to the Ministry of Oceans and Fisheries.

China expressed anger in October 2014 when a South Korean coast guardsman shot and killed the captain of a Chinese fishing vessel who had violently resisted the inspection of his ship for suspected illegal fishing. In 2011, a South Korean coast guard officer was killed in a clash with Chinese fishermen in South Korean waters.

The waters off the Korean Peninsula's western coast have also seen violent clashes between the rival Koreas because Pyongyang doesn't recognize the sea boundary unilaterally drawn by the American-led U.N. command at the end of the 1950-53 Korean War.

North Korea last month threatened to fire without warning at South Korean warships if they crossed into its waters, a day after the South's navy fired warning shots to chase away two North Korean ships that briefly crossed the boundary. The countries have also fought three bloody naval skirmishes in the area since 1999.