TOKYO - Japan's Prime Minister Yukio Hatoyama on Friday pledged more than 500 billion yen (US$5.5b, S$7.7b) in loans over three years for five Southeast Asian Mekong River nations, an official said.

Hatoyama made the announcement when he spoke with his counterparts from Cambodia, Laos, Myanmar, Thailand and Vietnam at the start of a two-day meeting in Tokyo, Japan's first summit with the Mekong countries.

"Japan pledges for the next three years more than 500 billion yen in ODA (overseas development assistance)," Hatoyama told the visiting leaders, a Japanese government spokesman told reporters.

Most of the aid would be low-interest yen-denominated loans, including for transregional highway projects, the Asahi newspaper reported in its evening edition.

The report also said Japan's aid to military-ruled Myanmar would continue to be limited to humanitarian assistance.

Much of the region along the lower stretches of the 4,800-kilometre (2,980-mile) Mekong River was long isolated by war and turmoil, and remains poorer than other parts of Southeast Asia.

Development has picked up recently, with Japan, China and multilateral lenders bringing investment, including for a highway network.