Japanese Defense Minister Takeshi Iwaya said Friday he told his Chinese counterpart Wei Fenghe that unilateral action is unacceptable in the South China Sea, where Beijing is expanding its military presence.

After the launch in June of a bilateral communications mechanism to avert unwanted clashes at sea and in the air, the Japanese and Chinese defense chiefs also discussed deeper exchanges between defense authorities, according to Iwaya.

The meeting, the first official one since November 2015, was viewed as the latest sign of warming bilateral relations, coming a week before Japanese Prime Minister Shinzo Abe's scheduled talks with Chinese President Xi Jinping in Beijing.

"In light of mutual understanding and confidence building, defense exchanges are very important," Iwaya told Wei at the outset of the meeting in Singapore. "I'd like to take this opportunity to pave the way for a full-blown improvement of Japan-China defense exchanges."

Despite the recent thaw in diplomatic ties, Japan has been keeping close tabs on China's maritime assertiveness. In September, a Japanese Maritime Self-Defense Force submarine joined a drill in the South China Sea for the first time.

During Friday's meeting on the sidelines of a regional gathering of defense chiefs, Iwaya is also believed to have sought Chinese cooperation in establishing a hotline that would enable direct communications between defense authorities.

Following many years of strained ties over a small group of Japanese-controlled, Chinese-claimed uninhabited islands in the East China Sea as well as wartime history, the June launch of the communications mechanism was seen as a major step forward in avoiding unwanted clashes between the two Asian countries.

But they did not specify the geographical coverage of the mechanism and sidestepped the territorial row over the Senkaku Islands that are known as Diaoyu in China.

Chinese vessels have repeatedly entered Japanese waters around the Senkakus, with the most recent intrusion on Wednesday.