Leaders from China, Cambodia, Laos, Myanmar, Vietnam and Thailand will gather in Phnom Penh for the second meeting of the Lancang-Mekong Cooperation group today.
At the second summit, the member countries are expected to approve the Phnom Penh Declaration and the five-year Action Plan for Mekong-Lancang cooperation in 2018-2022.
Over the past two years, more than 12,000 students from the Mekong nations received scholarships from the Chinese government.
The world's 12th-longest river, the Mekong runs almost 5,000 kilometers (3,100 miles) from the Tibetan Plateau down into mainland Southeast Asia before emptying into the South China Sea in Vietnam.
Cambodian Prime Minister Hun Sen, facing Western donor pressure over a crackdown on critics ahead of 2018 elections, is seeking more aid and investment from China.
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.
"Cambodia is a country that is geographically important in the 21st Century Maritime Silk Road as well as an important partner in the Lancang-Mekong Cooperation mechanism", said Gu Jiayun, director of the Center for Cambodian Studies at Beijing Foreign Studies University.
At the joint press conference, the Chinese premier said the sub-regional cooperation forum is not a mere talk shop but an opportunity to actually get things done.
The biennial MLC Leaders' Meeting is organised in rotation among the member countries.
With the theme "Peaceful river and our sustainable development", the summit will review cooperation among the countries over the last two years, discuss future cooperation orientations, and hear reports from working groups. Many said great progress has been made in a short time. "They are really committed and deliver what they have said or promised".
With control over the headwaters of the river - known there as the Lancang - Beijing can dam its section of the river while the impacts are felt downstream.
Calling the two countries close neighbors with profound friendship, Li said China always respects Cambodia's sovereignty and independence, and supports the country to stick to a development path with Cambodian characteristics.
The stated goal of the forum is to and boost the quality of life for the millions living in the Mekong sub-region.
The river is important to Southeast Asia, where an estimated 60 million people rely on the river and its tributaries for food, water and transport. The river's basin is home to up to 1,700 fish species, making it the most diverse basin after the Amazon and Congo.
Yet the river is also another potential source of regional tensions due to an increasing number of hydroelectric projects that are altering its flow and raising concerns of ecological damage.
China is the driving force behind numerous projects, having already built eight dams on the river since the 1990s and is now building or planning more than a dozen more. So far seven megadams have been built and more than 20 are under construction or planned in Yunnan, Tibet and Qinghai, according to International Rivers, a US-based non-profit environmental and human rights organisation.
The project is set to start this year and would take at least 44 months to complete, he added.
They also pledged to work together to handle the issue of legal status of Cambodians of Vietnamese descent in Cambodia in line with Cambodia's law and the spirit of the bilateral friendly neighbourliness.