The Trump administration announced on Monday that it was moving to greatly reduce the number of Central American immigrants who would be able to seek asylum in the United States by changing the laws regarding who is eligible for protection consideration.

The rule came into effect on Tuesday and was immediately challenged by the American Civil Liberties Union as unconstitutional.

The new rule prevents those who pass through another country without first seeking asylum there for seeking asylum in the United States. This will prevent many of the immigrants from Honduras, Guatemala and El Salvador from seeing protection in the United States if they did not first do so in Mexico.

“Until Congress can act, this interim rule will help reduce a major ‘pull’ factor driving irregular migration to the United States and enable DHS and DOJ to more quickly and efficiently process cases originating from the southern border, leading to fewer individuals transiting through Mexico on a dangerous journey,” acting Homeland Security Secretary Kevin McAleenan said in a statement.

The change has not been agreed to so far by Mexico, but such concerns have not stopped the Trump administration before. The United States was in the process of negotiating a similar deal with Guatemalan President Jimmy Morales on Sunday, but the deal was cut short when the Guatemalan Constitutional Court ruled against it.

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There are three exceptions to this new asylum rule, according to the statement released by the Departments of Justice and Homeland Security. The rule allows for immigrants who sought asylum in a third party country and were denied, immigrants who could show they were the “victim of a severe form of trafficking in persons,” and immigrants who only came to the United States through a country that does not comply with the international laws surrounding treatment of refugees.