Two big silver and gold mines and a copper mine owned by Carlos Slim, one of the world's richest men, have been closed by labor conflicts, the company said Monday.

The Minera Frisco company said its El Coronel mine in the north-central state of Zacatecas has been shuttered by workers during a conflict between two rival unions, one of which holds the contract at the mine and another seeking to organize workers there.

Julio Pomar, a spokesman for the combative National Mine Workers' Union, said workers at the El Coronel open-pit mine had decided to switch allegiance from a pro-government union to his group.

Pomar claimed that over the weekend Frisco fired more than 200 of the workers in retaliation for switching allegiance. In Mexico, companies often prefer docile pro-government "front" unions that have little shop-floor presence, almost never strike and sign lenient contracts that favor employers.

Slim's spokesman, Arturo Elias Ayub, denied that Frisco had fired anyone, even though the wildcat stoppage in which workers have blocked access to the mine is not recognized under Mexican labor law and could merit dismissal.

"We have not fired anyone at all," Elias Ayub said. "That is totally false."

"This is a dispute between two unions, and the company is caught in the middle," he said. "We are not involved in that. It is up to the employees to choose their union."

Elias Ayub also denied that the labor group currently representing workers was a "front" union.

"It is one of the most serious unions in the country," he said.

El Coronel is one of the most important mines in Mexico. It produced 197,631 ounces of gold and 20,419 ounces of silver in 2011.

Elias Ayub said that two other mines, the San Francisco del Oro gold and silver mine and the Maria copper mine in northern Mexico, had also been shuttered by stoppages.

He said those are recognized strikes over contract renewals. Frisco's other six mines were operating normally, the company said.

Pomar said the other strikes included concerns over working conditions in addition to wages and benefit issues.

Discontent with conservative, pro-government unions is growing. On Monday, a group of workers at the Mexican plant of Japanese automaker Honda Motor Co. announced it is trying to form an independent union, but claimed the company had fired their supporters to break the organizing effort.

The United Honda Workers Union said the company dismissed 11 workers at Honda's plant in the western state of Jalisco. The union hopes to get an organizing vote among the workers at the plant.

Honda's Mexico office did not immediately respond to requests for comment.

In the past, President Enrique Pena Nieto's Institutional Revolutionary Party, the PRI, depended on the pro-government unions for political support. But Pena Nieto claims his party has changed, and in February the government arrested the head of the country's powerful teachers union on embezzlement charges. However, that union leader, Elba Esther Gordillo, had broken politically with the PRI years before.