Japan, a longtime supporter of economic development in Southeast Asia, is stepping up its engagement in the region as China has raised its profile in the 10-member Association of Southeast Asian Nations (ASEAN).
Tokyo is also enhancing its role in Southeast Asia's security debates, especially the South China Sea, where it said the disputes need to be settled by the rule of law and without the use of force.
Earlier this week, Japan announced a three-year, $6.8 billion program to boost regional infrastructure across the Greater Mekong Subregion, which includes parts of Vietnam, Laos and Thailand.
Speaking at a university in Bangkok, Japanese Foreign Minister Fumio Kishida said the Japan-Mekong Connectivity Initiative aims to "create a framework" that supports efforts to improve infrastructure such as bridges and rail networks, human resource development and support for the Mekong River region.
Efforts in region
Japan's latest program for the Mekong River region comes after a 2013 package that provided 2 trillion yen in Official Development Assistance (ODA) to ASEAN countries over five years. Japan also backs ASEAN regional integration with a $100 million fund.
Thai political scientist Thitinan Pongsudhirak said Japan's long-standing programs of development assistance and diplomatic support is under increasing pressure by China's growing regional influence.
"Foreign Minister Kishida's visit is designed to emphasize and regain lost ground to say that Japan is still here – it wants to come back here – it's not going to forfeit its long-term investment on the various projects in the past years in mainland Southeast Asia," Thitinan said.
In March, China hosted the Lancang-Mekong cooperation summit in Hainan province, seen by analysts as the latest effort by Beijing to increase ties with countries in the Greater Mekong Subregion – notably Cambodia, Laos, Thailand and Vietnam -- through infrastructure spending.
The Asian Development Bank (ADB), with long-standing support from Japan, has played a key role in providing development assistance funds for infrastructure in the region.
Japan, as well as United States and Canada, did not join the China-backed Asian Infrastructure Investment Bank (AIIB).
China is keen to highlight its role in regional development through the Lancang-Mekong Cooperation program, political scientist Thitinan said.
"[China] is going to be the only game in town now for mainland Southeast Asia – apart from the Greater Mekong Subregion (GMS) – irrespective of the [intergovernmental] Mekong River Commission. So China will not play by the rules unless it is set up on its own. So these rules have to be China's rules and the LMC [Lancang-Mekong Cooperation] is going to be that vehicle; that platform," he said.
"Japan is reassessing its role. It has the financial muscle now with some new development finance package and these countries in mainland Southeast Asia are very important to Japan," Thitinan added.
He said for regional countries, Japan's higher-profile role offers a counterbalance to avoid being "beholden to and subject to China's whims and budget interest."
In his speech earlier this week, Foreign Minister Kishida was also looking to wider regional security issues, especially over the increasingly disputed waters of the South China Sea.
In his address at a Bangkok university, Kishida said "peace and stability" were a prerequisite for economic prosperity, including the underlying principle of the "rule of law," highlighted by "maritime security."
Kishida called for the establishment of a code of conduct, with its aim of settling disputes over the South China Sea.
Japan has increased maritime support for both Vietnam and the Philippines in steps seen as Japan's "maritime pivot" to Southeast Asia.
Vietnam and the Philippines, along with Malaysia and Indonesia, have conflicting maritime claims in the South China Sea with Beijing, which claims almost all of the area as its sovereign territory.
But Carl Thayer, a defense analyst at Australia's University of New South Wales, said China tries to exclude outsiders from negotiations to manage the security of the region.
Japan has to act to avoid being isolated, Thayer said.
"China is feeling some pushback, particularly led by Japan that's trying to stiffen regional countries. But without decisive American action, Japan is just not strong enough to continue to do it," he said. "Bottom line is that time seems to be on China's side."