SEOUL, South Korea >> South Korea sent military vessels to repel Chinese fishing boats that were illegally harvesting prized blue crabs near the disputed sea boundary between the Koreas before the fishermen retreated Friday, South Korean officials said.

Four naval and marine boats entered neutral waters around South Korea’s Ganghwa island to chase away about 10 Chinese boats, which by afternoon had escaped into North Korea-controlled waters, said a Defense Ministry official who didn’t want to be named, citing office rules.

The operation was approved by the United Nations Command that governs the zone where fishing activity is prohibited.

Depending on weather and water conditions, the operation will resume Saturday and continue until the Chinese boats withdraw further, the Defense Ministry official said. The South Korean military and maritime police personnel who carried out the mission were accompanied by translators and two monitors from United Nations Command.

“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 before the operation started and the Chinese boats were warned in English and Chinese, said the Defense Ministry official, who didn’t provide further details about the operation.

Days earlier, South Korean fishermen towed away two Chinese fishing boats catching crabs south of the sea boundary and handed them over to local South Korean authorities. North Korea said after that incident that South Korean fishing and naval vessels had invaded their territory.

Chinese fishing boats have been going farther afield to feed growing domestic demand for seafood as catches have decreased in waters close to China’s shores. 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.

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 2014 when a South Korean coast guardsman shot and killed a Chinese boat captain 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 western waters off the Korean Peninsula have also seen violent clashes between the 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.

The countries have fought three bloody naval skirmishes in the area since 1999, and last month North Korea threatened to fire at South Korean warships if they entered its waters, after the South’s navy fired warning shots to chase away two North Korean ships that crossed the boundary.