US authorities are testing a river in Arizona for possible cross-border contamination from a toxic mine spill in northwestern Mexico, an official said Tuesday.

The checks came after a massive acid leak in the Sonora River from the Buenavista copper mine, the worst environmental disaster on record in Mexico's mining industry.

The spill turned a 60-kilometer (40-mile) stretch of the waterway orange and caused Mexican authorities to shut off the municipal water supply to 20,000 people in seven towns.

"Two water-quality inspectors from the Arizona Department of Environmental Quality were dispatched today to the border region where the San Pedro River crosses into Arizona from Sonora," said a spokesman for the Arizona watchdog.

The International Boundary and Water Commission later said experts had not found any visual signs of pollution, but explained test results would take longer.

"The team observed no anomalies in the San Pedro River and took water quality samples from both the tributary and the river. Lab results will not be available for several days," said spokeswoman Sally Spener.

The San Pedro River flows north from Mexico, entering the United States to the west of Naco, Arizona. The Buenavista mine is located approximately 30 miles (48 kilometers) upstream from the US-Mexico border.

The IBWC is responsible for applying the boundary and water treaties between the two countries.