Marcelo Ebrard, Mexico’s top diplomat, said at a news conference in Mexico City that there was an understanding that both sides would evaluate the flow of migrants in the coming months. If the number of migrants crossing the United States border is not significantly reduced, he said, both sides have agreed to renew discussions about more aggressive changes to regional asylum rules that could have a bigger effect.

“Let’s have a deadline to see if what we have works, and if not, then we will sit down and look at the measures you propose and those that we propose,” Mr. Ebrard said, describing the understanding reached by negotiators last week…

American officials said Monday that what Mr. Trump appeared to be referring to was the agreement in principle to revisit the migration situation, and they said it gave the United States strong leverage over Mexico to live up to its promises. The numbers will be reviewed in 45 days and again in 90 days, officials said.