PHNOM PENH (Reuters) - Countries along the Mekong River said on Wednesday they would reinforce security, trade and environmental cooperation in the face of concerns over competing demands on its waters, particularly given multiple dams being built by China.

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The 4,350 km (2,700-mile) long Mekong, known as the Lancang in its upper reaches, flows from China along the borders of Myanmar, Laos and Thailand through Cambodia and Vietnam, where it has supported farming and fishing communities for millennia.

Following an inaugural meeting of the Lancang Mekong Cooperation (MLC) forum in 2016, its member countries met for a second time in Phnom Penh at talks chaired jointly by China - an ever more dominant force in the region - and host nation Cambodia.

“Political and security cooperation is of crucial importance to the MLC initiative,” they said in a statement. Cooperation on trade, transport tourism and environmental protection was also vital.

Despite the show of unity between the Mekong leaders, campaign group International Rivers said there were major concerns about the future of the river, with 20 dams planned on the upper Mekong in addition to eight already built.

“Over the past two decades, China’s upstream dams have altered the river’s natural flood-drought cycle and blocked the transport of sediment, drastically affecting ecosystems and fisheries downstream,” said Maureen Harris, its Southeast Asia program director.

“Effective regional cooperation is urgently needed on the Mekong to ensure its sustainable development and management.”

Communities in Cambodia are as reliant as any on the river.

But the government is a strong backer of China’s positions and has won billions of dollars in Chinese aid and investment as well as political support at a time when it is facing criticism from Western donors for eliminating the main opposition party.

In an article in the Khmer Times newspaper, Chinese premier Li Keqiang said the Mekong meeting and his official visit on Thursday would further strengthen the relationship.