There's a feeling Donald Trump's threat to impose a 100 percent tariff on any and all movies made outside the United States is unworkable.

The US president didn't specify when or how it could be enacted in his post on social media - which followed an initial threat on foreign-produced movies in May.

Screen Production and Development Association President Irene Gardiner says there's confusion about what he means.

"It seems to be that the problem he's trying to solve is American productions not shooting in America. So the way to fix that is their own incentives."

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