COMITAN, Mexico — Mexican authorities stepped up monitoring along well-traveled routes for migrants in southern Mexico over the weekend, checking identifications, pulling migrants off public transport and intercepting four trucks packed with nearly 800 migrants.

The National Migration Institute said 1,000 immigration agents had been deployed in the north and south of Mexico. The deployment comes as Mexico faces heightened pressure from the U.S. to reduce the surge of mostly Central American migrants through its territory. Mexico plans to position 6,000 National Guard troops by Tuesday to its southern border with Guatemala.

The Associated Press saw nearly 10 armed soldiers at a checkpoint near Ciudad Cuauhtemoc, in Chiapas state, wearing black armbands to indicate they are part of the National Guard. The soldiers stopped vehicles while immigration officials checked identification and removed passengers without documents. At another checkpoint just north of Comitan in Chiapas, more than a dozen apparent National Guardsmen drove around backroads in the rain and dark, looking for migrants.

In the Gulf coast state of Veracruz, the National Migration Institute said 791 people were taken Saturday to a migration facility and that drivers of the tractor-trailer trucks transporting them were arrested.

Migrants are routinely transported through Mexico in packed semis, sometimes in dangerous conditions without food or water or sufficient fresh air. Government video showed officials breaking the lock on the door of one cargo truck and helping migrants out.