Mexico has condemned new guidelines issued by the United States, under which almost all illegal immigrants can be subject to deportation.
The new rules include sending undocumented people to Mexico, even if they are not Mexicans.
But Mexican Foreign Minister Luis Videgaray says his country cannot "accept unilateral decisions imposed by one government on another."
Two top US officials are in Mexico to discuss the measures.
Secretary of State, Rex Tillerson, and the head of Homeland Security, John Kelly, will hold talks with President Enrique Pena Nieto, amid one of the most serious rifts between the two neighbours in recent years.
The changes announced on Tuesday include plans to enforce an existing provision of the US Immigration and Nationality Act that allows authorities to send illegal immigrants back to Mexico, regardless of where they are from.
But it is unclear whether the US has authority to force Mexico to accept foreigners.
The Obama government focussed on deporting immigrants convicted of serious crimes.
Now, the new priorities are broad enough to apply to almost any illegal immigrant, including anyone who has been charged with a crime, misrepresented themselves, poses a risk to public safety, or "abused any programme related to receipt of public benefits".
The new guidelines also allow Border Patrol and Immigration and Customs Enforcement to deport people immediately.
Expedited deportations can now be used against undocumented immigrants anywhere in the US, who are unable to prove they have been in the country for more than two years.
Previously, expedited removals were applied to people who had been in the country for less than two weeks and were within 100 miles of the border.
An estimated 11 million undocumented immigrants live in the US, many of them from Mexico.