Rio Tinto Alcan has locked out workers at its smelter in Alma, Que., after last-ditch contract talks collapsed Saturday and the union's contract expired at midnight.

The company announced the lockout early Sunday morning, saying plans were in place to ensure its aluminum operations continue to run safely and efficiently.

"We will work to limit the labour disruption's impact Rio Tinto Alcan's customers," the company said in a release.

Staff will continue to operate the plant during the lockout until further notice, the company said.

On Friday about 800 workers from three negotiating units rejected what the company said would be its final offer.

Union officials had a strike mandate before the talks collapsed and had warned that a work stoppage could be imminent.

The sides in the dispute haven't been able to agree on conditions related to subcontracted labour.

The Alma facility hosts one of Rio Tinto Alcan's most important North American aluminum smelters.