China's Mekong River dams held back large amounts of water during a damaging drought in downstream countries last year despite China having higher-than-average water levels upstream, a US research company said in a study.

China's government disputed the findings, saying there was low rainfall during last year's monsoon season on its portion of the 4,350-km (2,700-mile) river.

The findings by Eyes on Earth Inc, a research and consulting company specialising in water, published in a US-government funded study, could complicate tricky discussions between China and other Mekong countries on how to manage the river that supports 60 million people as it flows past Laos, Myanmar, Thailand and through Cambodia and Vietnam.

Last year's drought, which saw the Lower Mekong at its lowest levels in more than 50 years, devastated farmers and fishermen and saw the massive river recede to expose sandbanks along some stretches and at others turned from its usual murky brown to bright blue.

"If the Chinese are stating that they were not contributing to the drought, the data does not support that position,” said Alan Basist, a meteorologist and president of Eyes on Earth, which conducted the study with funding from the US State Department's Lower Mekong Initiative.

Instead, satellite measurements of “surface wetness” in China’s Yunnan province, through which the Upper Mekong flows, suggest the region in 2019 actually had slightly above-average combined rainfall and snowmelt during the May to October wet season.

But water levels measured downstream from China along the Thai-Lao border were at times up to 3 metres (10 feet) lower than they should have been, the group said in the study.

That suggests China is "not letting the water out during the wet season, even when the restriction of water from China has a severe impact of the drought experienced downstream," Mr Basist said.