A crash involving two police buses and a truck left at least 12 people dead and 48 injured in the Argentine province of Chubut, according to an official list of fatalities.
The victims in the crash that took place on Tuesday include nine police officers and the three drivers, according to local officials.
"These are people who gave their lives doing their job," said the provincial governor of Chubut, Martin Buzzi.
Police were sent to the province to help subdue a strike of some 400 workers at the Pan American Energy, a company co-owned by British, Chinese and Argentine interests, where acts of vandalism had been reported.
Chubut is home to one of the most important oil reserves in the province, and produces about 40 per cent of Argentina's petrol.
President Cristina Kirchner announced an unspecified period of national mourning for the accident victims.
"Many of these officers don't make a third or a fourth as much as the people who are occupying (the plant) and committing acts of vandalism," she said.