Prime Minister Shinzo Abe unveiled the pledge at a summit with his counterparts from Cambodia, Laos, Myanmar, Thailand and Vietnam -- fast-growing economies through which the lower section of the Mekong river flows.
"Japan will implement support worth around 750 billion yen (USD 6.1 billion) in official development assistance for the next three years," Abe told a news conference following the seventh annual Japan-Mekong summit.
"The Mekong region, which has vast demand for infrastructure, is one of our most important areas," Abe said.
It was not immediately clear if the pledge included previously-earmarked Japanese financial assistance, or whether it was made up entirely of newly-allocated funds.
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"The Mekong region is the most dynamic economic centre, but there still is room for huge growth," Thai Prime Minister Prayut Chan-O-Cha told the news conference.
The meeting came as the leading Nikkei business daily said Friday that three Japanese companies had secured an order worth over 32 billion baht (USD 947 million) to equip a railway linking the Thai capital Bangkok with nearby suburbs.
In a separate deal, Japan, Thailand and Myanmar signed an accord for a Special Economic Zone (SEZ) in Dawei, southeastern Myanmar, local media said.
When completed, the zone on the Andaman Sea coast will have a total area of 200 square kilometres, making it one of the largest SEZs in Southeast Asia and a gateway for the Mekong region's trade with India, the Middle East and Africa, Kyodo News said.
Abe has upped efforts to sell highways, train systems and power plants around the world, a key element in his bid to bolster the economy and Japan's standing abroad.