Mexico has angrily reacted to what one official called "hostile" new US immigration guidelines ahead of talks with senior Trump administration envoys on the volatile issue.
The US Department of Homeland Security unveiled plans to consider almost all illegal immigrants subject to deportation, and will seek to send many of them to Mexico if they entered the United States from there, regardless of nationality.
The tension over the timing of the rules mirrors an outcry when President Donald Trump said on Twitter that Mexico should pay for his planned border wall shortly before Mexican President Enrique Pena Nieto was due at a Washington summit in January.
However, White House spokesman Sean Spicer said in Washington US-Mexico ties were "phenomenal right now" and that he expected a "great discussion." Other senior officials also put on a brave face, telling reporters the trip was aimed at building a close working relationship.
Trump, who took office last month, campaigned on a pledge to get tougher on the estimated 11 million illegal immigrants in the US, playing on fears of violent crime while promising to build the wall and stop potential terrorists from entering the country.
US Secretary of State Rex Tillerson landed in Mexico City on Wednesday afternoon. He was joined by Homeland Security Secretary John Kelly for talks the White House said would "walk through" the implementation of Trump's immigration orders.
Kelly signed the guidelines issued by his department on Monday.
Mexico's lead negotiator with the Trump administration, Foreign Minister Luis Videgaray, said there was no way Mexico would accept the new rules, which among other things seek to deport non-Mexicans to Mexico.
"I want to say clearly and emphatically that the government of Mexico and the Mexican people do not have to accept provisions that one government unilaterally wants to impose on the other," Videgaray told reporters at the Foreign Ministry.
He said the issue would dominate the talks on Wednesday and Thursday. Mexico will insist that the United States proves the nationality of any person it wants to deport to Mexico, he said.
"We also have control of our borders and we will exercise it fully," he said, adding that Mexico was prepared to go the United Nations to defend the freedoms and rights of Mexicans under international law.
Roberto Campa, who heads the human rights department of the Interior Ministry, said the plan to deport non-Mexicans to Mexico was "hostile" and "unacceptable."
Another potential point of friction is a review ordered by Trump of the US aid Mexico receives, dominated by $US2.6 billion ($A3.4 billion) allocated largely for security under the Merida Initiative. Some believe the order carries a threat of cutting off such support.
