MEXICO CITY – Grupo Mexico said Wednesday it legally resumed operations at its San Martin mine in 2018 after a decade-long strike, countering a Mexican union that recently submitted a petition to US labor officials over alleged worker rights violations.

The US government last week asked Mexico to investigate the matter under the United States-Mexico-Canada Agreement (USMCA) trade pact, citing the request from the Mexican union, known as The Miners, and two major US labor organizations.

The groups argued that Grupo Mexico's operations at the San Martin mine in Zacatecas state, which has lead, zinc and copper, violate a strike that began in 2007, and that the company has bargained with employees who did not have the right to formally represent the mine's workforce.

Operations resumed in 2018 when, according to Grupo Mexico, a majority of workers voted to go back to their jobs and withdrew support for the striking union.

Mexican labor authorities recognized that vote as valid, the company added in its Wednesday filing with the country's stock exchange.

"Lifting the strike ... was a result of the free and majority decision of workers," Grupo Mexico said in the statement.

The U.S. Trade Representative in a letter to Mexican officials on Friday said it was concerned workers were being denied the rights to free association and collective bargaining.

"It appears that the Facility is engaging in normal production during an ongoing strike, without waiting for appropriate authorization from the Mexican courts," the letter said.

"The strike movement and its demands are totally legal," The Miners union said in a statement on Friday following the US complaint.

Mexico's government had 10 days from Friday to determine whether to accept the case for review, and has not yet commented on the matter.

Since operations resumed, the San Martin mining unit has received more than $105-million in investments, produces 1.25-million tons of minerals each year and generates nearly 1 300 jobs, Grupo Mexico said.

The case is the 11th US labor complaint in Mexico under the 2020 trade pact, which also aims to improve workplace conditions throughout North America.