BUENOS AIRES: 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, swarmed with emergency workers.

Nine people were rescued and being treated in local clinics. “The bus fell about 18 metres,” 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.

The cause of the pre dawn accident, in which the bus carrying 60 police officers aboard plunged some 15 meters (50 feet) into a dry riverbed, was unclear, said Gustavo Solis, mayor of Rosario de la Frontera in northern Salta province.

For reasons still unknown, the driver lost control of the bus as it started to cross a bridge and it tumbled into the ravine, police said in a statement.

Solis said the road where the accident occurred is known to be in poor condition.

“Those of us who know the area try to avoid driving at night,” Solis said.

Border security has become a hot issue in Argentina as the country has emerged as part of a route used for smuggling Andean cocaine to Europe and for human traffickers sending Syrian refugees to the Western Hemisphere.

Some press reports estimated as many as 40 people were killed, and rescue crews were working to free others who were trapped.

President Mauricio Macri, elected last month on a platform that included improving Argentina's rural roads, sent his condolences to 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.