MEXICO CITY

Mexico has cooperated to significantly slow the flow of migrants. The new White House chief of staff insists it is not enough. Since President Trump resumed office, the number of illegal crossings at the U.S.-Mexico border has dropped to the lowest point in decades. Mr. Miller’s goal, they said, is zero crossings.

Mexican officials have said that, with a nearly 2,000-mile border, that is simply not possible. One of the main forums for Mr. Miller’s demands has been his near daily calls with the U.S. ambassador to Mexico, Ronald ​Johnson, some of the officials said. On calls with staff, Mr.

Miller, previously the deputy chief of staff, frequently rails about the number of border crossings, despite their low figures, the officials said. That has been especially true in recent months, they said, as border crossings increased slightly, to the highest point in Mr. Trump’s second term. Thank you for your patience while we verify access. If you are in Reader mode please exit and log into your Times account, or subscribe for all of The Times.