US President Donald Trump has described a convoy of thousands of migrants from Central America as a "national emergency".
Writing on Twitter, Mr Trump vowed to cut US aid to three Central American countries over a migrant caravan heading to the US southern border.
Thousands of migrants are advancing towards the USA through Mexico and so far efforts to stop them at the Mexican border have failed.
Mr Trump wrote: "It looks like Mexico's Police and Military are unable to stop the Caravan heading to the Southern border of the United States.
"Criminals and unknown Middle Easterners are mixed in."
In another tweet, he said: "Guatemala, Honduras and El Salvador were not able to do the job of stopping people from leaving their country and coming illegally to the US.
"We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them."
Guatemala Honduras and El Salvador combined received more than $500 million (£382 million) in funding from the US in the last financial year.
Overnight, thousands of people bedded down on rain-soaked sidewalks, benches and public plazas in the southern Mexico city of Tapachula.
The migrant caravan, which started out more than a week ago with less than 200 participants, has drawn additional people along the way and it swelled to an estimated 5,000 on Sunday after many migrants found ways to cross from Guatemala into southern Mexico as police blocked the official crossing point.
Mr Trump's comments marked the latest escalation by the US president, who is seeking to re-inject immigration politics into the national conversation in the closing weeks of the American mid-term elections.
On a three-day campaign visit to western states last week, Mr Trump raised alarm over thousands of migrants travelling through Mexico to the US and threatened to seal off the Mexican border if they are not stopped.
A Pentagon spokesman, Army Lt Col Jamie Davis, said the Pentagon has received no new orders to provide troops for border security.