"Having a common market, having the European Union (EU)-type of arrangements ... It's not something that will happen right away but certainly something we should work towards," the ADB's Mekong Department director general, Rajat Nag, told AFP in an interview.

"Countries have to think in the context of the subregion and the region" for collective economic growth, he said.

Japan's biggest-selling daily the Yomiuri Shimbun daily has reported Tokyo will propose the creation of an East Asian community or an Asian version of the European Union at the commemorative summit meeting here between Japan and ASEAN on December 11-12.

Nag was in Tokyo for a seminar on Friday to drum up Japanese private-sector investment in new members of the Association of Southeast Asian Nations (ASEAN), namely Cambodia, Laos, Myanmar and Vietnam.

The ADB is aiming to raise 14 billion dollars by the end of the decade for 11 flagship projects for the Greater Mekong Subregion development.

They cover areas such as telecommunications, energy, transport, tourism and other forms of infrastructure and services along the Mekong and aim to help narrow the economic gap between old and new ASEAN members.

The river, extending for 4,000 kilometres (2,400 miles), is among the world's longest and least spoiled, passing through China, Myanmar, Thailand, Laos, Cambodia and Vietnam.

Nag said the new ASEAN countries needed to assure foreign investors about their political and economic environments as well as the existence of a fair dispute-resolution mechanism.

However, Toyota vice president Yoshio Ishizaka told AFP after the Tokyo seminar: "The sudden change of the rules of the game in Vietman has somewhat dampened the automotive industry's eagerness to invest there."

He was referring to a series of planned tax hikes aimed at encouraging Vietnamese-based vehicle manufacturers, who include Toyota and Isuzu, to increase their local content ratios.

"I think they need consistency and transparency" to attract more investment, Ishizaka said.

To garner Japanese investors to the communist nation, Vietnam on Friday signed an agreement which would guarantee Japan access to its tightly controlled markets.

Vo Hong Phuc, Vietnam's minister of planning and investment, said he expected the accord would encourage Japanese companies to invest in his country.

An official from a Japanese trading house welcomed the pact as it "widens investment options."