Former US President Donald Trump has doubled down on his hardline posture toward China by threatening to impose "a 100-percent tariff" on cars made in Mexico by Chinese firms.

Trump spoke during a rally in Ohio on Saturday, days after he had cemented his position as the presumptive Republican nominee for the November presidential election.

Trump said Chinese firms are building "big monster car manufacturing plants" in Mexico and that they think they are going to sell cars to the US while not hiring Americans.

He warned that he would, if elected president, "put a 100-percent tariff on every single car that comes across the line."

The tariff level is twice what Trump had pledged against such Chinese vehicles in an interview with a US business news channel, CNBC, earlier this month.

A growing number of Chinese automakers have been constructing production plants in Mexico. The trade deal currently in place in North America allows tariff-free vehicle shipments from Mexico to the US if they meet strict conditions.

Analysts say Trump is seeking to woo support by showing his intention to address concerns in the US automobile industry that lower-cost Chinese imports may threaten domestic jobs.