A bus carrying Argentine border patrol officers crashed into a ravine in the northern province of Salta on Monday, killing 42 people, while nine were being treated for injuries, provincial emergency official Francisco Marinaro told local television.
"We have 42 dead so far," he said in a TV interview from the scene, where an overturned bus was shown, surrounded by emergency workers.
Nine people were rescued and being treated in local clinics.
"The bus fell about 18 meters [59 feet]," Marinaro said.
The vehicle was part of a three-bus convoy carrying officers of Argentina's gendarmerie, which patrols the country's borders. Salta borders Bolivia, Chile and Paraguay.
Passengers on the other two buses were the first people on site to attempt to save the officers trapped in the wreckage, officials said.
Border security has become a hot issue in Argentina as the country is situated on a route used for smuggling Andean cocaine to Europe and for human traffickers.
President Mauricio Macri, elected last month on a platform that included improving Argentina's rural roads, sent his condolences to the families of the victims of the crash.
"We need to improve our highways so these things don't keep happening," he told reporters on the sidelines of a news conference in Buenos Aires province.