By Bernie Woodall and Allison Lampert

DETROIT (Reuters) - United Auto Workers President Dennis Williams said on Monday the U.S. union will support Canada's unionized auto workers during contract talks with General Motors Co but declined to say whether the UAW would refuse work at GM plants in the event of a Canadian strike.

Williams said in an interview he believes GM and Jerry Dias, president of the Canadian union Unifor, will “come to a conclusion that’s best for all parties” and hopes they can reach a new four-year contract without a strike.

Dias and Unifor have threatened the strike of about 4,000 workers at GM plants in Ontario unless a new four-year contract is reached by Sept. 19.

“The UAW is going to support the Canadian auto workers," said Williams. Williams met with Dias on Sunday.

A strike would have ripple effects at U.S. plants, particularly if Unifor workers walk off their jobs at an engine and transmission plant in St Catharines.

Williams would not comment on whether the UAW would refuse extra work to make up for lost output if Unifor's GM workers strike at two Ontario plants.

“I don’t want to comment on that because I don’t know what it will bring us, but we’ll do whatever we can to help (Unifor) in any way we can," Williams said.

If GM asks for extra work at the UAW's plants, "we’ll have discussions with General Motors and tell them how dissatisfied we are."

Existing contracts expire Sept. 19 for Canadian GM, Fiat Chrysler Automobiles and Ford Motor Co workers. Dias picked GM as its "strike target," meaning the union will try to get a deal with GM and use it as an outline for deals at the others.

Dias recently said he expected the UAW "would respect any action" Unifor takes and not replace production from GM's St Catharines factory.

If Unifor acts, "striking at St. Catharines would provide the most harm to GM," wrote Sam Fiorani, vice president global for AutoForecast Solutions in a recent article. "However, General Motors maintains other powertrain facilities outside of Canada which, with little investment, could absorb the St. Catharines volume."

Buzz Hargrove, former president of Unifor's predecessor, the Canadian Auto Workers, says he remembers how employees at GM's Oshawa, Ontario plant refused to produce extra parts for the automaker during a 1998 UAW strike.

    "Given that history, I think that they (the UAW) would be reciprocal," Hargrove said.

(Reporting by Bernie Woodall in Detroit and Allison Lampert in Montreal; Additional reporting by Susan Taylor in Toronto; Editing by Chris Reese and Cynthia Osterman)