Chinese President Xi Jinping has vowed not to stir up trouble in the South China Sea but said China would react "in the necessary way" to provocations by other countries, the official Xinhua news agency reported.
The comments come at a time of deep tension between China and Vietnam over Beijing's decision in early May to move an oil rig into disputed waters between the Paracel islands and the Vietnamese coast.
Days after China deployed the rig, the Philippines accused Beijing of reclaiming land on a disputed reef in the Spratlys to build what would be its first airstrip in the South China Sea.
"We will never stir up trouble, but will react in the necessary way to the provocations of countries involved," Xinhua quoted Xi late on Friday as saying in a meeting with Prime Minister Najib Razak of Malaysia, which is also embroiled in a long-running maritime dispute with China.
China has become increasingly willing and able to assert its claims over disputed waters, causing concern among the other parties to the disputes, analysts say.
The decision to deploy the oil rig enraged Vietnam and sparked anti-China rioting. Scores of Vietnamese and Chinese ships continue to square off around the rig and a Vietnamese boat sank this week after a collision that both sides blamed on the other.
Xi told Najib the situation in the South China Sea was "stable in general, but signs deserving our attention have also emerged".
Southeast Asian nations with maritime claims have been slow to band together against China, but last week Vietnamese Prime Minister Nguyen Tan Dung and Philippine President Benigno Aquino made a rare joint denunciation of China.