A list of rebalancing measures is being prepared says EU Commissioner Cecilia Malmstrom (Reuters)

The European Union (EU) will retaliate if US President Donald Trump imposes "disastrous" tariffs on automobiles, EU trade commissioner Cecilia Malmstrom warned Thursday.

"We are preparing a list of rebalancing measures," Malmstrom said, as Washington studies imposing duties on cars following its tariffs on aluminium and steel.

"Similar moves on cars would be disastrous," the Swedish commissioner told a conference in Brussels.

Her warning comes before her boss, European Commission Chief Jean-Claude Juncker, travels to Washington next Wednesday to try to persuade President Trump to drop tariffs on metals and avoid duties on cars.

"Tariff measures on cars are neither wanted or warranted," Malmstrom said.

"They are at best a solution in search of a problem. At worst they are an illegal move to gain leverage in trade negotiations."

Trump imposed duties of 25 percent on steel imports and 10 percent on aluminium in March, but had given temporary exemptions to the EU and some other close trading partners as talks continued.

Those exemptions were lifted in early June, prompting a raft of retaliatory tariffs by the EU, and Trump is now considering whether to impose hefty levies on European vehicles.

German Chancellor Angela Merkel said last week that she would back opening talks with trading partners on lowering automobile duties, in what appeared to be an olive branch to Trump.

Washington has shown no sign of compromise as it pursues its "America First" policies across a range of issues, even if it means alienating traditional allies.

Besides the tariffs, the US has threatened European companies with huge fines if they continue to operate in Iran, after Trump reimposed far-reaching sanctions while pulling out of the landmark 2015 deal on curbing Tehran's nuclear programme.

Earlier this month Trump implemented tariffs on $34 billion worth of Chinese imports, drawing a tit-for-tat response from Beijing.