US President Donald Trump has threatened to use military force to completely close the US-Mexico border.
The warning, via a series of tweets on Thursday, comes as a new caravan of more than 3,000 migrants from El Salvador, Honduras and Guatemala heads north.
Mr Trump has already threatened to cut all aid payments to the three countries over illegal immigration.
The group says they are fleeing violence and poverty.
“In addition to stopping all payments to these countries, which seem to have almost no control over their population, I must, in the strongest of terms, ask Mexico to stop this onslaught – and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!,” Mr Trump wrote.
The president has previously threatened to cut Honduran aid. The US sent more than $175m (£130m) to the country in 2016 and 2017, according to the US Agency for International Development.