Japan wants to work with countries in the lower Mekong river basin and will help them improve infrastructure and bolster development with 750 billion yen (US$7 billion) in aid over three years, its foreign minister said on Monday.
Foreign Minister Fumio Kishida made the pledge to help the Southeast Asian economies in Bangkok, where on Sunday he began a week-long visit to the region in which Japan competes with China for influence.
"Japan would like to work with the countries of the Mekong region to create a framework to support efforts by the Mekong countries in a detailed manner, on a region-by-region basis or on a theme-by-theme basis," Mr Kishida said in a speech at Chulalongkorn University.
Japan announced the three-year plan last year.
China has offered billions of dollars in infrastructure loans and government aid programmes to Southeast Asian countries.
Mr Kishida did not mention China in his speech. He is also due to visit Myanmar, Laos and Vietnam.
On Monday, he met Prime Minister Prayut Chan-o-cha. Thailand has drawn closer to China since the coup which many western countries criticised.
Mr Kishida and Gen Prayut discussed Thailand's political process, regional terrorism threats and economic challenges, a Japanese official said.
Mr Kishida visited Beijing on the weekend where both China and Japan expressed willingness to improve relations strained over conflicting territorial claims in the East China Sea.
In his speech in Bangkok, Mr Kishida addressed maritime security and renewed a call for countries to respect the rule of law.
He also backed a Southeast Asian bid to draft a code of conduct for the South China Sea, where China's claim to virtually the entire sea clashes with claims to parts of it by Vietnam, Brunei, Malaysia and the Philippines.
"We must establish a regional order whereby the principle of the rule of law is truly upheld and practiced," he said. "I would like to renew my call for the early conclusion of an effective Code of Conduct in the South China Sea."
Japanese Prime Minister Shinto Abe is pursuing a more robust foreign policy but Masato Otaka, deputy press secretary at Japan's Ministry of Foreign Affairs, told reporters Mr Kishida's visit was not aimed at counteracting China's influence.
On Sunday, Mr Kishida reaffirmed Japan's economic ties with Thailand, an important base for many Japanese companies, after Japanese investment in the country nosedived in 2015.