Mexican officials detained nearly 800 undocumented migrants on Saturday, the government said, in one of the biggest swoops against illegal immigration in recent months, as members of the National Guard began patrolling the southern border.

Mexico's National Migration Institute (INM) said in a statement late on Saturday that 791 foreign nationals were found in four trucks stopped in the eastern state of Veracruz, confirming earlier reports about a mass detention.

The apprehension came as Mexico steps up efforts to reduce a surge of migrants toward the U.S. border under pressure from U.S. President Donald Trump, who vowed to hit Mexican goods with tariffs if Mexico does not do more to stem illegal immigration.

As part of those efforts, Mexico has pledged to deploy 6,000 National Guard members along its border with Guatemala.

Although there have been few signs of that deployment so far, a Reuters reporter near the border in Tapachula this weekend saw a handful of security officials wearing National Guard insignia and spoke to others in military outfits who said they were part of the guard.

Mexico made a deal on June 7 with the United States to avert the tariffs, setting the clock ticking on a 45-day period for the Mexican government to make palpable progress in reducing the numbers of people trying to cross the U.S. border illegally.

There has been a jump in apprehensions at the U.S.-Mexico border this year, angering Trump, who has made reducing illegal immigration one of his signature policy pledges.

Most of those caught attempting to enter the United States are people fleeing poverty and violence in three troubled Central American nations, Guatemala, Honduras and El Salvador.

Mexico's decision to tighten its border and respond to Trump's threats has caused tensions within the government, and on Friday, the head of the INM, Tonatiuh Guillen, resigned.

He was replaced by Francisco Garduno, who had previously served as the head of Mexico's prison system. (Reporting by by Roberto Ramirez in Tapachula and Dave Graham in Mexico City Editing by Nick Zieminski and Sonya Hepinstall)