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.
Why does it concern Trump?
The group of Central American migrants has made its intentions clear: they are heading to the US.
Since he was on the campaign trail, Mr Trump has lambasted illegal immigrants, and this latest caravan comes after a major crackdown on migrants heading over the Mexican border.
Changes to detention rules saw thousands of migrant children detained and separated from their parents earlier this year, sparking national and international condemnation.
The president's threats also come just weeks before the mid-term elections on 6 November, which could see Democrats unseating Republicans on Capitol Hill.
Mr Trump's recent comments are probably an effort to bolster support among his base by cracking down on migration.
What will happen to the people?
The Associated Press reports that many migrants do not have passports and have been using national ID cards, which allow them to travel within Central America. Mexico, however, requires a passport at entry.
Human rights groups have criticised the US and Mexican response to the caravan.
Mexican president-elect Andrés Manuel López Obrador plans to offer work visas to Central Americans when he takes office in December.
"It is a plan that we have, that anyone who wants to work in Mexico will have a work visa," he said.
Why are they leaving?
An estimated 10% of the population of Guatemala, El Salvador and Honduras have fled danger, forced gang recruitment and dismal economic opportunities.
The region has one of the highest murder rates in the world. The UN reported murder rates in 2015 in Honduras standing at 63.75 deaths per 100,000 and El Salvador at 108.64 deaths.