In a statement, the United Steelworkers union (USW) said an armed group's killing of two strikers at the Canadian-owned Media Luna gold mine in Mexico on Saturday "underscores the widespread repression of basic labor rights."

The mine in the state of Guerrero is owned by Canada's Torex Gold Resources.

In early November its workers went on strike to demand the right to form a union of their choosing, after Torex set up what the USW called an illegal "protection union" that doesn't legitimately represent workers.

On Saturday, following an escalation of tensions, an armed group attacked the strikers at a roadblock near the mine, and two brothers, Victor and Marcelino Sahuanitla Pena, were killed.

"We are once again urging the Canadian government to intervene with Mexican authorities and the company to recognize the basic rights of Mexican workers and prevent further violence," said the USW's Ken Neumann.

"The Mexican government and this Canadian company must ensure this conflict is resolved without further bloodshed," he said.

Canada is in the midst of talks with Mexico and the United States to revamp the North American Free Trade Agreement.

One of Canada's proposals, which has the backing of the USW, was to amend the pact to include stronger labor provisions.