United Nations Command vessels patrol the Han River Estuary, which marks the eastern most portion of the border between North and South Korea, in this undated photo.

SEOUL, South Korea - South Korean forces in speedboats and the U.S.-led command launched a rare operation against illegal fishing by Chinese boats in the crab-rich waters near the North Korean border.

The crackdown began Friday after several Chinese boats were captured by the local coast guard and a group of South Korean fishermen who decided to take matters into their own hands.

South Korea’s coast guard said it seized a 50-ton Chinese boat that allegedly caught about 100 pounds of crabs Saturday off the Yellow Sea island of Yeonpyeong. Seven sailors were arrested, according to the Yonhap news agency.

The top U.S. commander in Korea, Gen. Vincent Brooks, who also leads the United Nations Command, authorized the operation to enforce restrictions on fishermen in the estuary where the Han River meets the Yellow Sea, according to a statement.

It said the intrusions of unauthorized fishing boats violated the 1953 armistice that ended the three-year Korean War and left the peninsula divided. Some 28,500 American servicemembers are stationed in South Korea.

“United Nations Command takes its responsibility to maintain the armistice very seriously. We had a responsibility to act and we are doing that,” Brooks said in the statement released Friday.

The problem is exacerbated by the sensitive location of the waters, which are teeming with blue crabs and other marine life. South Korean patrol boats are strictly limited in their ability to chase illegal trawlers due to fears of provoking the North. Clashes have broken out in the past.

Washington and Seoul also need China’s help in efforts to rein in North Korea’s nuclear weapons program. Beijing, a traditional ally of the North, signed off on a fresh round of tough U.N. sanctions earlier this year.

Officials said the operation, executed by maritime civil police and representatives of the U.N. Command military armistice commission, will continue as long as necessary.

It was the first time South Korea and the U.N. command have taken joint action to repel foreign vessels from the buffer zone, a South Korean military official told Yonhap.

The operation involved a recently formed military police team with 24 members and four speedboats, the official said.

The issue came to the fore after South Korean fishermen captured two Chinese boats just south of the border a week ago Sunday. They turned the crew and boats over to the coast guard, local media reported. The coast guard also captured two Chinese boats in the area Tuesday.

South Korea’s government registered a formal protest to China via diplomatic channels and called for a response, according to the Foreign Ministry.

The countries agreed last year to step up joint efforts to counter illegal fishing in South Korea. But South Korea says the problem has only increased.

“Illegal fishing by Chinese vessels is not only depleting and devastating the fishery resources in (the area) but also contributing to an increased potential for accidental military conflict between the South and the North,” the defense ministry said in a statement.

North Korea and China were notified ahead of the operation, it said.

The number of Chinese fishing boats illegally crossing into South Korean territorial waters rose from two to three cases a year to 120 in 2015 and 520 as of May, Yonhap reported.

South Korean fishermen say resources are being depleted and their livelihoods are threatened while they are forced to stand by and watch.

“Hundreds of them come and go for fishing almost every day. Many even dare to bring their boats alongside our piers,” said Park Tia Won, the head of a fishing union on Yeonpyeong Island.

Cho Chul-hui, a guide on the island, said the Chinese boats often use a common net to dredge all the fish in their path, a practice known as bull trawling.

“Their bull trawls are seriously damaging and deeply digging out the sea bed, destroying spawning grounds and the ecosystem,” he said.

gamel.kim@stripes.com

Twitter: @kimgamel