US President Donald Trump has threatened to impose US tariffs on $US11 billion ($A15 billion) worth of European Union products, heightening tensions over a long-running transatlantic aircraft subsidy dispute and opening a new front in his global trade war.
The United States and Europe have been locked in a years-long spat over mutual claims of illegal aid to plane giants, Netherlands-based Airbus and US-based Boeing, to help them gain advantage in the world jet business.
The case, which has been grinding its way through the World Trade Organisation for almost 15 years, is approaching the final stages of arbitration.
"The World Trade Organisation finds that the European Union subsidies to Airbus has adversely impacted the United States, which will now put Tariffs on $11 Billion of EU products!" Trump said on Twitter.
"The EU has taken advantage of the US on trade for many years. It will soon stop!"
Trump's public attack on the EU came even as his administration has yet to hammer out trade deals with China and neighbours Mexico and Canada.
A day earlier, the US Trade Representative proposed targeting a list of EU products for tariffs, ranging from large commercial aircraft to dairy products and wine, as retaliation for European aircraft subsidies. The final list is expected this (northern) summer.
In response, the European Commission said earlier on Tuesday it had started to draw up plans to retaliate over Boeing subsidies.
"The Commission is starting preparations so that the EU can promptly take action based on the arbitrator's decision on retaliation rights in this case," a Commission spokesman said.
"The European Union remains open for discussions with the United States, provided these are without preconditions and aim at a fair outcome."
Both sides have won partial victories in claiming Airbus and Boeing received unlawful subsidies but disagree on the amount involved and whether each has complied with earlier WTO rulings.
Airbus said it saw no legal basis for the US move and warned of deepening transatlantic trade tensions.
The EU is already facing US tariffs on its steel and aluminium exports and Trump has repeatedly threatened to hit EU cars with punitive duties.
French Finance Minister Bruno Le Maire told a conference in Paris that the two sides needed to reach a friendly agreement.
"When I see the situation global growth is in, I don't think we can afford to have a trade conflict, even if only on the specific issues of the aircraft industry in the United States and Europe," he said.
