Amid deep strains in the U.S. relationship with Mexico, a country that's been a favorite target of President Trump, Secretary of State Rex Tillerson and Homeland Security chief John Kelly took part in talks in Mexico City on Thursday that were aimed at smoothing out tensions."That's going to be a tough trip," the president saidThursday morning at the White House. Some of the key issues between the two countries: immigration, border security, trade and U.S. aid to Mexico.In a Feb. 21 executive order, President Trump expanded the category of unauthorized immigrants the U.S. can deport to Mexico, including asylum-seekers from other countries. Mexico's foreign minister, Luis Videgaray, said earlier this week that Mexico "will not accept" the U.S. immigration order.Speaking to reporters Thursday afternoon, Kelly sought to allay some of Mexico's concerns, saying twice, "There will be no — repeat, no — mass deportations." He added, "There will be no use of military force in immigration operations — none."Earlier in the day, in Washington, Trump had described deportations as "a military operation."Trump's Feb. 21 executive order also ordered construction of a U.S.-Mexico border wall. To help pay for the wall, the president has threatened to slash aid to Mexico, much of which the U.S. delivers via the Merida Initiative, a 10-year-old program whose primary purpose is to deter organized crime and bolster the rule of law.The wall wasn't mentioned at Thursday's press conference.Tillerson told reporters that Mexico and the U.S. need to "modernize and strengthen our trade and energy relationship," and Kelly emphasized the need for "safe and secure trade."Back in Washington, the president told a group of manufacturing CEOs the trade relationship with Mexico was "unsustainable.""We have $70 billion in deficits, trade deficits, and it's unsustainable," the president said. "We're not going to let it happen. Can't let it happen. We're going to have a good relationship with Mexico, I hope. And if we don't, we don't. But we can't let that happen."The U.S. Trade Representative says the trade deficit with Mexico is $58 billion, not $70 billion.The president has called NAFTA, the landmark 1994 deal eliminating most tariffs among the U.S., Mexico and Canada, "the worst trade deal in history" and a "terrible disaster." He has repeatedly vowed to renegotiate it — or "terminate" it.Under the tripartite pact's terms, the president can withdraw the U.S. unilaterally, without congressional approval, giving six months' notice.U.S. exports to Mexico, its third-biggest trading partner, have increased by 468 percent from 1993, the year before NAFTA was signed, according to the U.S. Trade Representative website, accounting for nearly 16 percent of all U.S. exports in 2015."NAFTA did not cause the huge job losses feared by the critics or the large economic gains predicted by supporters," a 2015 report by the nonpartisan Congressional Research Service concluded. "The net overall effect of NAFTA on the U.S. economy appears to have been relatively modest."But critics of NAFTA — including many U.S. unions, which opposed the deal from the start — argue that wages and employment have both suffered as a result of it. They've blamed it for trade deficits, job loss to Mexico, poor job opportunities, even illegal immigration.Treasury Secretary Steven Mnuchin said Thursday that he does not foresee changes anytime soon to NAFTA, and it's unclear what trade policies the Trump administration might pursue should there ever be a NAFTA withdrawal. But a pullout could lead to some of the very job losses Trump is trying to avoid, according to a recent report from the Center for Automotive Research."A wholesale withdrawal from NAFTA could set in motion a series of unintended consequences that would constrain future growth of the U.S. automotive industry," the research firm warns.Other possible unintended consequences: A loss of nearly 5 million U.S. jobs, according to the Wilson Center's Mexico Institute. And imposing huge tariffs on Mexico could push that country into recession — leading, perhaps, to greater immigration pressures on the U.S. Copyright 2017 NPR. To see more, visit http://www.npr.org/.