European Commission President Ursula von der Leyen was responding to Donald Trump’s announcement that starting February, a 10% import tax will be imposed on goods from eight European nations that have rallied around Denmark in the wake of his stepped up calls for the United States to take over the semi-autonomous Danish territory of Greenland.

“The European Union and the United States have agreed to a trade deal last July,” Von der Leyen said at the World Economic Forum in Davos, Switzerland. “And in politics as in business – a deal is a deal. And when friends shake hands, it must mean something.”

“We consider the people of the United States not just our allies, but our friends. And plunging us into a downward spiral would only aid the very adversaries we are both so committed to keeping out of the strategic landscape,” she added, the AP reported.

She vowed that the EU’s response “will be unflinching, united and proportional.”

Trump has insisted the US needs the territory for security reasons against possible threats from China and Russia.

The American leader’s threats have sparked outrage and a flurry of diplomatic activity across Europe, as leaders consider possible countermeasures, including retaliatory tariffs and the first-ever use of the European Union’s anti-coercion instrument.

The EU has three major economic tools it could use to pressure Washington: new tariffs, suspension of the US-EU trade deal, and the “trade bazooka” — the unofficial term for the bloc’s Anti-Coercion Instrument, which could sanction individuals or institutions found to be putting undue pressure on the EU.

Earlier Tuesday, Trump posted on social media that he had spoken with NATO Secretary General Mark Rutte. He said “I agreed to a meeting of the various parties in Davos, Switzerland,” which is hosting the World Economic Forum’s annual meeting this week.

Trump also posted a text message from Emmanuel Macron in which the French president suggested a meeting of members of the Group of Seven industrialized democracies in Paris after the Davos gathering.