Vietnamese President Tran Dai Quang warned on Tuesday there would be no winners in any armed conflict sparked by territorial disputes in the South China Sea.
Quang, who is on a state visit to Singapore, told a forum that recent developments there were threatening regional security.
The Vietnamese leader did not mention any country but there is growing unease over China's actions.
China claims most of the South China Sea. It has reclaimed reefs and built airstrips capable of hosting military equipment, sparking anger from competing claimants led by Vietnam and the Philippines.
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