China is taking control of the flow of Southeast Asia's most important river through a dam-building "spree", US Secretary of State Mike Pompeo warned Wednesday, as water levels along the Mekong reach record lows.
The famous waterway starts in China and twists south through parts of Thailand, Laos, Myanmar, Cambodia and Vietnam, feeding 60 million people through its basin and tributaries.
But environmental groups fear Beijing-backed dams straddling the river upstream will destroy fish stocks and allow China to manipulate water flows.
Pompeo said Chinese plans for the river, which include the blasting and dredging of waterbeds, represent "troubling trends".
"We have seen a spree in upstream dam building that concentrates control over downstream flows," he said in Bangkok at an Association of Southeast Asian Nations summit.
The meeting marked a decade since the launch of the Lower Mekong Initiative, a US-funded development program, and Pompeo used the opportunity to point out flaws in China's activity on the river.
He accused the Asian giant of operating extraterritorial river patrols and of pushing new rules that would weaken the Mekong River Commission, an intergovernmental group monitoring construction along the water.
The top US diplomat is in Bangkok to reaffirm his country's "Indo-Pacific" vision for a region increasingly under Chinese hegemony.
The Mekong has garnered little attention next to the US-China trade spat or the stalled negotiations over North Korea's nuclear program.
But severe drought in Thailand has put China's Mekong vision under rare global scrutiny.
Last month the Mekong River Commission said river levels in June and July had dropped to "among the lowest on record".
Beijing's foreign minister Wang Yi said Wednesday that China had released more water "in order to help Thailand".
"Thailand's suffering is China's suffering," he said.
Conservationists have also pointed to safety and environmental dangers posed by mega-dams in neighbouring Laos.
The human cost of the dam building frenzy was laid bare last year when a massive hydropower project collapsed in southern Laos, killing dozens.