UPDATE 1-Strike begins at Vale Inco's Ontario sites

* Vale Inco workers on strike at Sudbury, Port Colborne

* Union urges company to return to the negotiating table (Updates with current status of the mine)

TORONTO, July 13 (Reuters) - Unionized workers at Vale Inco's nickel mine in Sudbury and its nickel refinery in Port Colborne, both in Ontario, went on strike early on Monday, after rejecting the company's contract offer over the weekend.

"The strike started at midnight and workers were on the picket line just after midnight, both at Sudbury and Port Colborne," said Bob Gallagher, a spokesman for the United Steelworkers union.

The strike won't have any immediate impact on nickel output because Vale had closed the Sudbury mine through the end of July because of low nickel prices.

Negotiations between Vale Inco -- the nickel mining and processing division of Brazil's Companhia Vale do Rio Doce (VALE5.SA) -- and its Sudbury union broke down as the two sides failed to agree on bonuses, pensions and other issues.

Members of the USW's Local 6500 voted overwhelmingly to reject the company's contract offer on Saturday. The Local represents about 3,300 workers at the Sudbury site in northern Ontario, which is one of the world's largest nickel mines. [ID:nN11472287]

And about 120 workers of USW's Local 6200, which represents workers at the Port Colborne nickel plant in southern Ontario, rejected the offer on Sunday.

Miners at the Vale's Voisey's Bay nickel-copper operations in Labrador on Canada's East Coast have already voted to authorize a strike, which will likely take effect in early August.

Gallagher said the union was willing to go back to the negotiating table if the company will to come back and talk about possible compromises. Company officials were not immediately available for comment. (Reporting by Euan Rocha; editing by Peter Galloway)