state that borders the United States, the state Secretariat of Public Security said.

Given "the emergency situation, we recommend exercising caution" although "the situation is already under control," the secretariat said on Twitter.

The clashes occurred along the Monterrey-Nuevo Laredo highway near the Corona Bridge, close to a gas supply company called Gas del Norte.

Authorities also cancelled classes.

"Officials from the Regional Education Center inform us that classes at the basic level have been suspended to care for the safety of the student community," said Nuevo Laredo Mayor Carmen Lilia Canturosas, according to local media.