Image copyright Reuters Image caption The migrants were found concealed in the trailers
Mexican authorities have found 600 migrants hidden in two shipping containers at a cargo port in Veracruz state, in the south-eastern part of the country.
The group, from Central America, were expected to reach the US after risking the trip across Mexico.
More than 1,000 undocumented migrants have been found crammed into a truck driving from Ciudad Juarez to the southern state of Tabasco.
Their planned destination was planned as Nashville, Tennessee.
Thirteen migrants were injured when the 32-tonne truck’s wheels buckled under the weight of the migrants and caught fire.
Mexican authorities said in a statement on Friday that they found the men, women and children “comprised of eight families with two adults in each family”.
The country’s National Migration Institute said it was “acutely worried” about the group’s welfare and asked for international assistance.
Mexico has had serious problems with illegal migration from Central America, which has increased recently as migrants fleeing violence in El Salvador, Guatemala and Honduras flee gang violence.
US President Donald Trump has announced plans to block funding to Central American countries unless they help stop the flow of migrants.