Mexico’s government says it will send 6,000 troops to its southern border with Guatemala in a crackdown on Central American migrants hoping to seek asylum in the U.S. Thursday’s announcement came as heavily armed Mexican soldiers and police blocked a caravan of about a thousand migrants as they walked a highway in the southern state of Chiapas. The crackdown came ahead of a threat by President Trump to impose 5% tariffs on all Mexican imports next Monday unless Mexico further tightens controls on immigration. Mexican officials are reportedly nearing a deal with the White House that would require asylum seekers to seek refuge in the countries they first cross into. Under the plan, Guatemalan migrants could only apply for asylum in Mexico; Hondurans and Salvadorans would be forced to apply as refugees in Guatemala. The emerging plan drew fire from civil liberties groups including the ACLU, which said such a change to the asylum system violates both U.S. and international laws and is unlikely to survive a legal challenge.