Mexico has sent hundreds of police to its southern border as a caravan of thousands of Central American migrants approaches from Guatemala.
The move came after President Donald Trump threatened to use military force to completely close the US-Mexico border over the issue.
He also said he aid could be cut to countries allowing the caravan to pass.
The group of Salvadoreans, Hondurans and Guatemalans say they are fleeing violence and poverty.
Mr Trump 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.
Where is the caravan now?
The migrant caravan is making its way through Guatemala, mostly on foot, with several of the quickest already at Mexico's southern border.
Their journey began in San Pedro Sula, Honduras, last week.
Most migrants are carrying few belongings, taking what they can carry in backpacks, as they head out on the nearly 2,800 mile (4,500km) trek.
Could Trump close the US-Mexican border?
Thousands of people and goods legally cross the US-Mexico border daily.
While Mr Trump has sent national guard troops to the border before, it is unclear what he means by shutting it down entirely, and whether that would affect businesses or people with legitimate visas.
And according to international law, the US cannot deport asylum seekers without first determining the validity of their claim.
Under pressure from the US to stop the migrants, Mexico has sent federal police to the border, though they are not officially there to stop the caravan.
Mexican officials said on Wednesday that those without papers would have to apply for refugee status or turn back.
US Secretary of State Mike Pompeo will be in Mexico this Friday, and US media report that he will discuss plans to stop the caravan.
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.
Erika Guevara-Rosas of Amnesty International said in a statement: "Mexican authorities should not take a Trump approach treating people like a security threat."
"These families deserve dignity and respect to ensure that no one is illegally returned to situations where they could risk serious harm due to violence."
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.
However, his incoming Foreign Minister, Marcelo Ebrard, said those without a visa would have to apply for refugee status.
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.
Jari Dixon, an opposition politician in Honduras, tweeted on Monday that the caravan was not "seeking the American dream" but "fleeing the Honduras nightmare".