The South Korean military started a five-day naval exercise Thursday near its disputed maritime border with the communist north, a local report said.

The country’s largest-ever anti-submarine drills are to be held near the location of the deadly sinking of a South Korean corvette in March, which Seoul blames on its northern neighbour, South Korean agency Yonhap News reported. The manoeuvres are planned to mobilize 4,500 troops from all branches of the armed forces, an amphibious landing ship, a submarine, a destroyer and around 50 fighter jets, the report said, citing the Joint Chiefs of Staff. Underwater firing drills were scheduled to take place near Baengnyeong Island, close to the Northern Limit Line which represents the maritime section of the border with North Korea, the report said.

The island is near the spot where the Cheonan corvette was sunk on March 26 with the loss of 46 lives. A South Korean—led investigation concluded that the explosion which sank the vessel was probably caused by a North Korean—made torpedo. Pyongyang has repeatedly denied any involvement in the incident, and requested that its own experts be given access to the recovered wreckage. The exercises in the Yellow Sea come less than two weeks after joint manoeuvres between the U.S. and South Korean navies in the waters off the other side of the Korean Peninsula, intended as a strong signal of deterrence to the North. North Korean on Tuesday threatened a “powerful physical retaliation” to the latest round of exercises. It made similar threats to the joint manoeuvres in July.

The Northern Limit Line was drawn up by the United Nations during the armistice which ended the 1950—53 conflict between the Koreas, but which has not resulted in a formal peace agreement. North Korea claims that the line is too far north.

The disputed maritime border has been the site of clashes between the two countries’ naval forces in 1999, 2002 and 2009.

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