A "bold" plan for a railway system connecting more than 300 million people who live around one of the world's great rivers, the Mekong, was approved Friday, officials said.

Ministers from Cambodia, China, Laos, Myanmar, Thailand and Vietnam adopted the plan which they called "a significant first step toward the development of an integrated... railway system".

The six nations' national railway systems do not link up except for a line that connects China and Vietnam, and Laos has no rail network at all.

The plan cites four possible ways of connecting the railways but it says the most viable route would stretch from Bangkok to Phnom Penh, then Ho Chi Minh City and Hanoi, and finally up to Nanning and Kunming, largely using existing lines or those already under construction.

"We think it's realistic to do one of the routes by 2020," said Lawrence Greenwood, a vice-president with the Asian Development Bank (ADB).

"Having said that, it is certainly bold and ambitious," he told reporters.

The only missing link on that route would be between Ho Chi Minh City and Phnom Penh, the 25-page railway plan says, estimating a cost of 1.09 billion dollars for completion.

This does not include roughly seven billion dollars in additional funding needed to upgrade the existing lines.

By 2025, an estimated 3.2 million passengers and 23 million tonnes of freight are forecast for the completed route, the document says.

The goal coincides with an effort by Mekong nations to develop "economic corridors" around new road links, which would help to reduce poverty and would be complemented by railway connections, the ADB said.

Creating the corridors of investment and development will require a smoothing of procedures for cross-border transport and trade, the ministers said in a joint statement.

Business leaders and other experts have said there are still too many bureaucratic hurdles to a free flow of regional goods.

Greenwood said ministers at the Greater Mekong Sub-region (GMS) conference "very importantly" agreed on a plan to smooth cross-border movement, which is supported by six million dollars in funding from Australia.

GMS is an ADB-supported programme that began 18 years ago to promote development through closer economic links between Cambodia, Laos, Myanmar, Vietnam and Thailand, as well as China's Yunnan province and the Chinese Guangxi Zhuang Autonomous Region.

Cambodian minister Cham Prasidh said the rail link would be a cheap way of transporting goods to the Mekong nations and beyond, to other members of the Association of Southeast Asian Nations (ASEAN).

But he said that while the nations around the Mekong are tightening transport and other links they have neglected the region's very heart -- the river itself.

Cham Prasidh, Senior Minister and Minister of Commerce, said the potential of the 4,800-kilometre (2,976-mile) river has not been fully tapped as the region develops economic corridors, which he likened to arteries.

"But we forget the heart and the Mekong River is the heart. We need to develop the heart first," he told AFP after making his suggestion to the conference.

Vo Hong Phuc, Vietnam's Minister of Planning and Investment who co-chaired the meeting with Greenwood, said ADB was asked to prepare a working group "to appropriately assess the use of the Mekong River."

But he said water resources are under the jurisdiction of another body, the Mekong River Commission.

Although they are growing fast, the Mekong nations -- except for Thailand -- have the lowest per capita gross domestic product among the 10 ASEAN members.