Water supplies to 20,000 residents of seven Mexican towns were cut on Tuesday after sulfuric acid leaked from copper mine in the northwestern state of Sonora.

Forty thousand cubic meters of the toxic chemical leaked into the Sonora river as a result of a "structural defect" at the Buenavista Copper Mine, a spokesman for the region's civil protection agency, Jesus Arias, told AFP.

Arias said the mine in the town of Cananea "did not alert any authorities" and the leak only came to light 24 hours later when it was reported by one of the towns along the 420-kilometer (260-mile) river.

The chemical caused an orange stain along a 60-kilometer (37-mile) stretch of the river bordered by the towns of Arizpe, Banamichi, Vabiacora, Aconchi, Cananea, Ures and Hermosillo.

Mexican authorities responded by shutting off the water supply to the towns and erecting barriers to seal the river off from the mine, which belongs to Grupo Mexico, a leading Latin American mining company.

Lime also was dumped into the river to neutralize the acidity of the leaked chemical.

Mexico's National Human Rights Commission said it was investigating the incident.

More than a quarter of Mexico's mining activity is located in the Sonora region, making it the country's top producer of gold, copper and graphite.