Shotaro Yachi, a special adviser to Prime Minister Shinzo Abe's Cabinet, said the most important diplomatic issue for Japan in the coming decades will be how to develop relations with China.

In a recent interview, the former vice foreign minister and the brain behind Abe's diplomatic policies stressed the need to hold talks "at various levels" to mend ties strained by the Senkakus dispute.

"China stands out as a very important country among the countries that are key to Japan's strategic diplomacy," Yachi reckoned.

"How to steer relations with China will be the biggest issue for Japan's diplomacy this century. It is important that we talk to each other calmly at various levels and refrain from provocative actions," he said.

With regard to a Chinese naval vessel's alleged use of weapon-targeting radar last month on a Japanese destroyer in the East China Sea, Yachi said the use of such radar "is an extremely dangerous act and violates a rule of the international community."

China's territorial claim over the Japan-administered Senkaku Islands, which has straining bilateral ties for months, "is not an argument that can be considered valid internationally," he added.

On the pending question of whether Japan will join the U.S.-led Trans-Pacific Partnership trade talks, Yachi said Japan should take part in the negotiations "at the earliest stage possible."

"We need to approach it from the viewpoint of sharing the benefit of the Asia-Pacific region's dynamic economic growth," he said.