The death toll from the accident involving a truck and two buses in the southern Argentine province of Chubut is officially at 12, with the majority of the victims members of the border patrol, police spokesmen said.
Early reports from the official Telam news agency put the death toll at 14 based on information provided by provincial prosecutors.
The accident killed nine border patrol officers and the drivers of the three vehicles, the government of Chubut said.
The border patrol officers were aboard one of the buses involved in the accident Tuesday on Highway 3 near Puerto Madryn, a popular Patagonian tourist destination located about 1,300 kilometers (808 miles) south of Buenos Aires.
Of the 48 people injured in the accident, 14 are being treated at hospitals and clinics in Puerto Madryn, the provincial government said.
Chubut Gov. Martin Buzzi has declared three days of mourning over the accident, whose cause is being investigated.
President Cristina Fernandez declared a period of national mourning during a ceremony at which she read the names of the dead officers.
The accident "is a tragedy that puts all Argentines in mourning," Fernandez said.
The border patrol officers had been deployed in the past few days to deal with the protests at the Cerro Dragon oil complex operated by Pan American Energy, Argentina's second-largest oil producer, which is owned by British, Chinese and Argentine interests.
Officials are negotiating with the protesters.
Fernandez criticized the decision by judicial officials to send the officers to the oil facility instead of using provincial officers. EFE