Matamoros, Mexico - Mexican soldiers rushed Thursday to set up emergency shelters near the border with the US ahead of President Donald Trump's threatened mass deportations.

The Mexican government said it planned to open nine shelters for its citizens and three more for deported foreigners, without clarifying the total capacity, under a scheme called "Mexico embraces you."

Navy personnel erected olive-green tents on a large sports field in the northeastern city of Matamoros, across the border from Brownsville, Texas, according to an AFP correspondent.

That shelter – one of three being built in Tamaulipas state – will be able to accommodate around 3,000 people, according to municipal authorities.

Mexico Mexico warns against US "invasion" and threatens fresh legal action against gun manufacturers

President Claudia Sheinbaum said this week that Mexico would provide humanitarian assistance to deported migrants from other countries before repatriating them.

On his first day back in office on Monday, Trump declared a national emergency at the US southern border and vowed to deport "millions and millions" of migrants.

His administration said it would also reinstate a "Remain in Mexico" policy that prevailed during Trump's first presidency, under which people who apply to enter the US from Mexico must remain there until their application has been decided.

The White House has also halted an asylum program for people fleeing authoritarian regimes in Central and South America, leaving thousands of people stranded on the Mexican side of the border.

On Wednesday, Trump's office said he had ordered 1,500 more military personnel to the border.

Mexico's government announced in December that it would launch a mobile app with an alert button for migrants facing imminent detention in the US, where there are several million undocumented Mexicans.