He spoke Thursday in Thailand’s capital with his counterparts from countries through which the river flows, including Thailand, Cambodia, Laos and Vietnam.
Pompeo’s concerns include the building of upstream dams, plans to blast river beds, and the staging of river patrols outside Chinese borders.
He noted findings that the river has been at its lowest levels in a decade, which he described as a problem linked to China’s decision to shut off water upstream.
China has built 10 dams along the upper stretch of the Mekong, the part it calls the Langcang.
Critics have long warned that these actions can have disruptive environmental and economic effects downstream.