EU chief Ursula von der Leyen said Tuesday the bloc plans "massive" investments in Greenland, as it seeks to counter US President Donald Trump's threats to take the autonomous Danish territory in the name of securing the Arctic.

"We are working on a massive European investment surge in Greenland," von der Leyen told the World Economic Forum in Davos, without giving specifics.

The European Commission chief pledged the bloc would "work with the United States and all partners on wider Arctic security," saying: "This is clearly in our shared interest, and we will step up our investment."

Short of providing figures, von der Leyen suggested the EU could direct part of its ongoing surge in defence spending on a "European icebreaker capability and other equipment vital to the Arctic security."

Von der Leyen warned the United States against hitting "long-standing allies" in Europe with punitive tariffs over Greenland -- recalling the transatlantic trade accord struck last year.

"In politics as in business -- a deal is a deal. And when friends shake hands, it must mean something," she said.

The EU chief called the proposed additional tariffs a "mistake" and vowed Europe's response to Trump's repeated threats would be "unflinching, united and proportional."

"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 warned.

EU leaders are to meet Thursday in Brussels for an extraordinary summit aimed at forging a united response to Trump's escalation over Greenland.