Li was speaking at a meeting of a forum in Phnom Penh on Wednesday with leaders of 5 countries along the Mekong River - Cambodia, Laos, Thailand, Vietnam and Myanmar.
The forum is seen as a rival to the Mekong River Commission, which has existed for more than 60 years but excludes China and Myanmar.
At the second Lancang-Mekong Cooperation (LMC) leaders' meeting here on Wednesday, Chinese Premier Li Keqiang announced that China will provide another 7 billion yuan (1.08 billion USA dollars) in government concessional loans to Myanmar, Laos, Thailand, Cambodia and Vietnam.
As such, China, which sits upstream, possesses considerable influence over the five downstream countries and has cemented that control by constructing dams that limit water required for downstream farming.
During the summit, Vietnamese Prime Minister Nguyen Xuan Phuc urged China to release sufficient water resources for the Lower Mekong countries, especially during dry season.
China will step up cooperation with the Mekong countries in prevention and control of infectious diseases, carry out cross-border projects on prevention and control of such diseases, and build a network for malaria elimination.
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Leaders of the six riparian countries held their first meeting in China in 2016 and the next summit is to take place in Laos in 2020.
At more than 45,000 kilometers long, the Mekong is the biggest river in Southeast Asia - and the most controversial.More news: Bruno Mars, The Killers and Muse Headline This Year's Bottle Rock
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The LMC mechanism has produced better-than-expected progress over the past two years as the countries moved forward with the principle of equality and inclusiveness and put development as a priority.
Led by Minister of Foreign Affairs Prak Sokhonn, Cambodia signed a new cooperation agreement with China in December, receiving more than US$7 million to fund various projects, including monitoring water quality, restoring forests and combating land degradation.
The heads of State and Government will debate major issues to develop the Mekong sub-region, focusing on their commitment to maintaining peace, stability, sustainable development and prosperity in that geographic zone of inestimable economic and ecological values.
Known as the Lancang in China, the Mekong River is vital to Southeast Asia, where more than 60 million people rely on it and its tributaries for food, water and transport. The basin is home to 1,200-1,700 fish species, making it the most diverse basin after the Amazon and Congo.
In exchange, Cambodia pledged to support China's worldwide goals, including its claims to disputed territory in the South China Sea.
China is the driving force behind numerous projects, having already built eight dams on the river since the 1990s and now building or planning more than a dozen more.
Beijing has already interrupted the Mekong's upper reaches with six dams, and is investing in more than half of the 11 dams planned further south, according to the non-profit group International Rivers.
China is also seeking to have parts or the river dredged or rapids cleared so that large cargo ships can navigate it. Environmentalists have warned this could have dire consequences on the ecosystem.
Overriding priorities should be given to hydro-meteorological statistics and information sharing, cooperation in response to droughts and floods and joint scientific research to construct reservoirs along the Mekong River as well as infrastructure development to facilitate flows of commodities, services and capital, and local people's travel, he noticed.