Prime Minister Yoshihide Suga, speaking in the Indonesian capital on Wednesday, also took a thinly veiled dig at China's activities in the disputed South China Sea.

Beijing has built and militarised manmade islands in the sea and is pressing its claim to virtually all of its key fisheries and waterways.

A number of Southeast Asian nations dispute China's claims to the waters.

"In the Indo-Pacific that connects maritime transport between Japan and ASEAN, we can achieve the peace and prosperity only by fully implementing rule of law that allows anyone freedom and openness," Suga told a news conference in Jakarta.

ASEAN is the 11-country Association of Southeast Asian Nations.

"But in the South China Sea, opposite actions are being taken and we are closely watching with grave concern," Suga said.

"Japan opposes any action that would increase tensions in the South China Sea."

Suga reiterated that all parties involved in the South China Sea disputes must make peaceful efforts toward resolution, not by force or threat.

Suga pledged to strengthen bilateral defence ties with Southeast Asian nations and promised further assistance to step up their maritime policing, including training and equipment transfer from the Japanese Coast Guard.

Suga chose Vietnam and Indonesia for his first overseas trip since succeeding Shinzo Abe as prime minister last month in order to demonstrate the region's importance to Japan.

Japan considers China's growing military activity to be a security threat and its defence papers have accused China of unilaterally changing the status quo in the South China Sea with its actions there.

Japan is also concerned about China's claim to the Japanese-controlled Senkaku Islands, called Diaoyu in China, in the East China Sea.

In both Indonesia and Vietnam, Suga has stressed that Southeast Asian countries are key to achieving what Japan terms a "Free and Open Indo-Pacific," a vision of economic and security cooperation between nations that was started by Abe as a way to counter China's growing might and influence.

Suga denied China's criticism that the initiative is an attempt to create a NATO in Asia.

"Achieving a rule-based free and open Indo-Pacific in this region contributes to the peace and prosperity for the entire world," Suga said.

The plan is not intended for special membership but "we can cooperate with any country that shares the view," he added.

Suga agreed with Indonesian President Joko Widodo to speed up ongoing negotiations for defence equipment and technology transfer.

Australian Associated Press