A four-year-old Venezuelan was killed and three other US-bound migrants were injured on Wednesday when a freight train they were riding atop derailed in northern Mexico, authorities said.

The boy's seven-year-old brother was hurt while his mother, 28, lost a foot and suffered a spinal injury in the accident, the health secretariat in Chihuahua state said.

A Nicaraguan migrant was also hurt, according to the national immigration institute.

The train overturned in a desert region where migrants often climb on the roofs of passing wagons.

Thousands of migrants fleeing violence and poverty travel across Mexico every year heading for the US border.

While some ride atop trains, others pay people smugglers to transport them in trucks or join caravans undertaking the long journey on foot.