Thieves who tapped an oil pipeline in northern Mexico caused a spill that contaminated the San Juan River, a key irrigation source for farmers, authorities said Thursday.

The crude from the Madero-Cadereyta pipeline polluted a 23-kilometer (14-mile) stretch of the river in Nuevo Leon state, said federal environmental protection agency Profepa.

"We are asking people in these areas to avoid drinking local water and also to avoid eating fish from the affected area," said Profepa's Victor Cabrera.

The spill happened Sunday when suspected members of a criminal gang tapped the pipeline to remove oil and sell it.

Crude seeped into the irrigation channels used by five towns, said Emeterio Arizpe Tellez, the mayor of the town of Cadereyta.

A clean-up team of 300 people from state energy company Pemex is mopping up the spill, but the operation will take about three months, he said.

Authorities say they will distribute bottled water to the affected towns.

The spill comes after a sulfuric acid leak at a copper mine in the northwestern state of Sonora on August 6 contaminated a river, forcing authorities to shut off the water supply to 20,000 people.