The Trump administration is moving to end asylum protections for most Central American migrants.
A rule published on Monday bars migrants from seeking asylum in the United States if they've travelled through another country first.
Tens of thousands of migrant families from Central America travel through Mexico to the US each month, many claiming asylum.
The Trump administration claims families are taking advantage of legal loopholes it says allow migrants a free pass to the country while they wait out phony asylum requests.
Attorney General William Bar said that the United States is "a generous country but is being completely overwhelmed" by the burdens associated with apprehending and processing hundreds of thousands of migrants at the southern border.
"This rule will decrease forum shopping by economic migrants and those who seek to exploit our asylum system to obtain entry to the United States," Mr Barr said in a statement.
The rule is almost certain to face a legal challenge.