JAKARTA, June 8 (Reuters) - The Southeast Asian bloc ASEAN will hold its first-ever joint military exercise in the South China Sea, its chair Indonesia said on Thursday, the latest multilateral security drills at a time of rising tension and uncertainty in the region.

The decision was taken at a meeting of military commanders of the 10-member Association of Southeast Asian Nations (ASEAN) in Indonesia, which will host the exercise in the North Natuna Sea, the southernmost waters of the South China Sea.

Indonesia's military chief, Admiral Yudo Margono, told state-run news agency Antara the exercise would be in September and would not include any combat operations training. The purpose, Margono said, was strengthening "ASEAN centrality".

ASEAN's unity has for years been tested by a rivalry between the United States and China that is being played out in the South China Sea. ASEAN members Vietnam, the Philippines, Brunei and Malaysia have competing claims with Beijing, which asserts sovereignty over vast stretches of ocean that include parts of Indonesia's exclusive economic zone (EEZ).

Indonesian military spokesperson Julius Widjojono said the exercise was related to the "high risk of disaster in Asia, especially Southeast Asia."

A conduit for about $3.5 trillion of annual ship-borne trade, the South China Sea has seen constant tension of late as China presses its claims with a huge deployment of coast guard and fishing boats as far as 1,500 km (932 miles) off its coastline.

China claims sovereignty via an expansive "nine-dash line" based on its historic maps, which an international arbitration court in 2016 ruled had no legal basis.

ASEAN has been pushing for a long-awaited maritime code of conduct with China to be completed and several of its members have had run-ins with Beijing in recent months.

Vietnam criticised China's deployment of a research vessel near several gas blocs in its EEZ, while Beijing was accused of sending suspected maritime militia into waters where navies of India and ASEAN countries held an exercise.

The Philippines chided China's coast guard for "dangerous manoeuvres" and "aggressive tactics" and plans to hold joint patrols with the United States, on top of an inaugural trilateral coast guard exercise they held with Japan this week.

China maintains its coast guard is performing regular operations in what is Chinese sovereign territory.