Abhisit, in a joint interview with Kyodo News and other media, also welcomed active involvement of China in the subregion, which comprises Thailand, Myanmar, Laos, Cambodia and Vietnam, as well as recent signs of "eagerness" of the United States to engage in development of the subregion.

Abhisit's remarks came ahead of his planned trip to Tokyo for an inaugural summit between Japan and five Mekong countries on Nov. 6-7 at the invitation of Japanese Prime Minister Yukio Hatoyama.

"Development of the (sub)region is also vital for a successful building of the ASEAN Economic Community," the Thai leader said. "It'll be a very good opportunity to give a push to make sure this process continues in a timely manner."

Abhisit acknowledged that the separate approaches to the Mekong subregion by China, and the United States "overlap," saying, "It can look untidy and it does mean a lot of meetings for officials."

"But it might be necessary at this stage," he said. "In many ways, the regional cooperation architecture has sort of evolutionary character to it -- it might not be tidy and neat, but it evolves naturally and serves its purposes."

He added, "I think once the integration program becomes clearer in terms of its path, then I think there will be further rationalization. For the time being, it does no harm."

China, through which the upper part of the Mekong River runs, has since 1992 engaged with the five downstream countries in a development project backed by the Asian Development Bank and known as the Greater Mekong Subregion scheme.

Chinese leaders have so far three summit meetings with leaders of the five countries -- in 2002, 2005 and 2008.

In July, U.S. Secretary of State Hillary Clinton held an unprecedented meeting with her counterparts from four of the five lower Mekong basin countries -- Cambodia, Laos, Thailand and Vietnam -- in which they agreed to cooperate in the areas of environment, health and education.

At the upcoming Japan-Mekong summit, Abhisit said, he will reaffirm Thailand's commitments to be a co-donor and partner for projects in such key areas as infrastructure, environment and human resources in the subregion.

Thailand has spent some $400 million in the last six years in various projects to develop infrastructure in the subregion, according to Abhisit, who added he expects Japan to extend assistance to further complete infrastructure development in needy areas.

He said he will also seek Japanese help on mitigating the effects of and adapting to climate change in the Mekong subregion.