Li made the pledge in his talks with Cambodian Prime Minister Samdech Techo Hun Sen in the Cambodian capital of Phnom Penh.
Amid the backdrop of the river's importance in connecting Europe through Southeast Asia and beyond in the grand infrastructure programme launched by President Xi Jinping, Chinese delegation leader Premier Li Keqiang will be looking to bolster China's influence over the Mekong region - as he faces his counterparts from Cambodia, Vietnam, Thailand, Laos and Myanmar.
Last week, Chinese vice foreign minister Kong Xuanyou said that in addition to being its largest source of worldwide aid, China is also Cambodia's largest source of foreign investment, its largest trade partner, and its largest source of foreign tourists. They reached consensus that the MLC should contribute more to promoting regional socio-economic development, narrowing development gap among member states and supporting the ASEAN Community building.
China is the largest source of foreign investment for Cambodia, Laos and Myanmar, and the third and fourth-biggest source for Thailand and Vietnam in 2017, respectively, according to China's Ministry of Commerce. Government leaders from the remaining four nations are also attending the meeting in Phnom Penh.
The biennial MLC Leaders' Meeting is organised in rotation among the member countries.
The meeting was themed "Our River of Peace and Sustainable Development".
The Mekong River, known as the Lancang in China, is the heart of mainland Southeast Asia. Some outstanding outcomes include implementing 130 projects, completing many "Early Harvest" projects, setting up centres for environmental, water resource and Mekong research cooperation, and establishing and operating the MLC Special Fund.
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However, junior trade minister Greg Hands said there was no geographical restriction that would prevent the U.K.'s participation. Michael Gove, the environment secretary, said in December that parliament would have an effective veto over any trade deal.
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.
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.
The administration of the river is also shared among all six countries, which are committed to deepening cooperation based on three pillars: politics-security, economy and sustainable development, and society-culture and people-to-people exchange.
Managing these competing demands is a hard job, but it's one that the Lancang-Mekong Cooperation is tackling head on.
Foreign ministers from the six countries through which the Mekong flows met in southwestern China last month to approve a draft of a five-year development plan for the river.
The regional superpower is now asserting its authority through the nascent Lancang-Mekong Cooperation forum, while appeasing its Southeast Asian neighbors with investment and soft loans.
The action plan states that "the Mekong-Lancang is moving towards a new sub-regional cooperation mechanism" that will in part support community-building and regional integration efforts of the Association of Southeast Asian Nations, which also includes Brunei, Indonesia, Malaysia, the Philippines and Singapore.