United States Ambassador to Mexico Roberta Jacobson is the latest in a series of senior foreign officials to resign as tension continues to mount between the two nations.
After two years of service with the embassy, Jacobson announced her resignation to staff members in a note Thursday, effective May 5.
"After more than 31 years of U.S. government service, I have come to the difficult decision that it is the right time to move on to new challenges and adventures," she wrote.
"This decision is all the more difficult because of my profound belief in the importance of the U.S.-Mexico relationship and knowledge that it is at a crucial moment."
Though Jacobson chose not to elaborate on the details behind her decision, she later tweeted that she would be pursuing other job opportunities and did not know who would replace her at the embassy.
Rumors say Trump plans to nominate former AT&T Chief Executive Ed Whitacre as the next ambassador to Mexico, the Mexican newspaper Reforma reports.
U.S. Secretary of State Rex Tillerson told his Mexican counterpart, Foreign Minister Luis Videgaray, in a February 17 phone call that Jacobson had resigned, the foreign ministry said in a statement.
"It is exclusively the job of the U.S. government to reveal publicly who it will name to lead its diplomatic representation in Mexico," the foreign ministry said, thanking Jacobson for her service in Mexico and across the region, and wishing her well.
Appointed by former President Barack Obama, Jacobson became ambassador to Mexico in May 2016, after serving as the State Department's Assistant Secretary of State for Western Hemisphere Affairs.
Many expected Trump to ditch Jacobson due to her links to the Obama administration and her often-vocal differences with the new U.S. president's proposals, including his border wall.
During her time as ambassador, however, Jacobson was generally well-regarded, Mexican diplomats told Reuters.
Other politicians stepping down include U.S. Ambassador to Panama John D. Feeley, Undersecretary of Political Affairs Tom Shannon, and William Brownfield, the former U.S. assistant secretary for International Narcotics and Law Enforcement Affairs.
Relations between the United States and Mexico have not been going well. Last week, Trump and Mexican President Enrique Peña Nieto postponed plans for Nieto's first visit to the White House, after a testy phone call involving Trump's border wall, a senior U.S. official said.
"The two leaders agreed now was not the immediate right time for a visit, but that they would have their teams continue to talk and work together," Reuters reported the official as saying.
Mexican officials had been talking about a summit between Trump and Peña Nieto in the next few weeks, without specifying when.
The Washington Post, which first reported the delay, said the two leaders spoke for about 50 minutes, but the discussion led to an impasse when Trump refused to publicly affirm Mexico's position that it would not fund construction of the wall along the U.S.-Mexico border.