DETROIT (Reuters) - United Auto Workers President Dennis Williams said on Thursday the union is contacting workers at Silicon Valley electric car maker Tesla Inc TSLA.O, and plans to boost efforts to convince U.S. consumers not to buy vehicles built in other countries, including those sold by the Detroit automakers.

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The UAW leader also used a meeting with reporters to praise President Donald Trump for calling on companies to produce more products in the United States, and promising to rework the North American Free Trade Agreement. But Williams said he disagreed with Trump’s order temporarily barring travel to the United States from seven Muslim-majority countries.

“It’s very dangerous to single out individual groups based on religion,” Williams said. “It’s un-American.” Some UAW members were stranded overseas by the ban before it was stayed by a federal appeals court, Williams said.

Williams’ comments highlight the political challenges facing U.S. labor leaders as they confront a Republican president who shares their skeptical views on free trade and values U.S. manufacturing, but whose policies on other issues are contrary to union principles.

“I’m interested in some of the things he’s doing,” Williams said. “I’m very concerned about some of the things he’s doing.”

Williams said he has not met with Trump, though other union leaders have.

The UAW leader endorsed Trump’s moves to pressure Detroit automakers to stop shipping vehicles into the United States from Mexico, and said the UAW is working on a new advertising campaign to encourage consumers not to buy foreign-made vehicles.

"If it's not made in America, don't buy it," Williams told reporters at a briefing at the union's Detroit headquarters, responding to a question about General Motors Co's GM.N Chevrolet Cruze hatchback, made in Mexico, and the Buick Envision sport utility, which GM imports from China.

“Boycotts may be coming back,” he said, adding he would prefer consumers buy a vehicle made by UAW workers, or a vehicle made in the United States by a foreign manufacturer.

Regarding Tesla, Williams reiterated the union’s denial last week that the union had paid a worker at the automaker’s Fremont, California, factory who went public with complaints about safety, pay and overtime. [nL1N1G1168]

UAW organizers are in contact with workers at Tesla, Williams said, but any formal effort by the union to organize workers will “be determined by the interest of employees.”

Tesla Chief Executive Elon Musk, in comments to the Gizmodo website, charged the UAW had paid the worker, and blamed the union for the failure of the GM-Toyota Motor Corp 7203.T joint venture that once operated at the factory, located on the east side of San Francisco Bay. [nL4N1FV4J4]

Tesla shares slid both before and after Williams’ comments on Thursday, and were down 3.9 percent at $268.90 in afternoon trading.