Li made the pledge in his talks with Cambodian Prime Minister Samdech Techo Hun Sen in the Cambodian capital of Phnom Penh.
The storied river, which twists through parts of Myanmar, Thailand, Laos, Cambodia and Vietnam, is the world's largest inland fishery and helps feed around 60 million people across its basin. It then flows through Myanmar, Laos, Thailand and Cambodia before entering the South China Sea through the delta region of southern Vietnam.
During his official visit in Phnom Penh, both sides inked agreements for Chinese loans for electricity and roadworks in Cambodia.
In the past year, China's economy has maintained a steady and favourable trend, with the overall situation better than expected, Li said at the Mekong-Lancang Cooperation Forum on Wednesday. Government leaders from the remaining four nations are also attending the meeting in Phnom Penh.
China is increasing its investment along the river through building dams and a navigation channel.
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.
The meeting was themed "Our River of Peace and Sustainable Development".
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Today's summit will further legitimise China's leadership of the Mekong region.
But environmentalists say the LMC aims to replace the long-standing Mekong River Commission - a regional body that has tried to manage development along the river - albeit without China.
The leaders adopted the Phnom Penh Declaration and the five-year MLC action plan for 2018-2022, which feature major orientations and measures for implementing cooperation in the three pillars of security-politics, economic and sustainable development, and society, culture and people-to-people exchange.
The biennial event is organised in rotation among the member countries.
He also announced the setup of a 5-billion-dollar credit line for supporting production capacity and equipment manufacturing cooperation among the Lancang-Mekong countries.
Speaking at the meeting, PM Phuc affirmed Vietnam's attachment of importance and contribution to MLC over the past years. After little more than two years of operation, the LMC has already brought Mekong countries closer together.
Mekong-Lancang collaboration focuses on management of hydric resources, connectivity, increased productivity, cross-border economic cooperation, agriculture, and poverty reduction. The Mekong-Lancang countries should be assisted to make use of opportunities from Industry 4.0 through sharing experience, knowledge and technology to promote industrialisation in the member nations; developing businesses and training skillful labourers; attracting resources into infrastructure development and creating conditions for projects to tap their potential and strength; providing jobs for people; enhancing cooperation in researching and transferring technology to develop agriculture, especially hi-tech organic farming.
Chinese Premier Li Keqiang pledged Thursday to build a community of shared future with Cambodia that is of strategic significance.