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MEXICO CITY (Reuters) - One person died and 81 others were injured on Tuesday when a truck crammed with Central American migrants flipped over in Southern Mexico on a highway that leads to the United States, according to officials in the Mexican state of Veracruz.

It was not immediately clear what caused the accident near the southeastern city of San Andres Tuxtla, about 515 kilometers (320 miles) from Mexico City, on the Carretera 180 highway snaking up to Texas.

“The majority (of the injured) were from Guatemala and others were Hondurans,” Mexico’s National Guard said in a statement, without providing detail on the severity of the injuries.

Human rights advocates have long warned that Mexico’s tougher stance on immigration was likely to drive migrants deeper into the shadows, forcing them to rely on smugglers and unsafe modes of transport.

Reuters photos from the site of the accident showed a white, medium-sized commercial truck overturned in a ditch, with the driver’s window crushed.

Mexico’s President Andres Manuel Lopez Obrador has tasked the National Guard militarized police with preventing migrants from reaching the United States amid fierce pressure from Washington to stem the flow of U.S.-bound migrants from Central America.

The majority of the injured migrants were taken to two nearby hospitals and a temporary shelter was established to treat some of the injured, Veracruz’s civil protection agency said in a statement.