BRUSSELS — U.S. President Donald Trump’s threats to annex Greenland and slap tariffs on its backers pose a challenge to Europe’s security, principles and prosperity, a top EU official said on Wednesday.

By Philip Blenkinsop BRUSSELS, Jan 21 (Reuters) - U.S. customs authorities will face a real challenge if President Donald Trump goes through with his threat to place tariffs on six EU countries - rather than the whole European Union - given the ease of movement of goods between EU members.

Trump said Monday that U.S. ownership of Greenland is "imperative."

In a post on social media, Trump said a 10% tariff will take effect on Feb. 1, and will climb to 25% on June 1 if a deal is not in place for the United States to purchase Greenland.

EU on Wednesday put a trade deal with the US on hold in response to Trump's repeated threats over seizing Greenland.

The European Union’s top official has described U_S_ President Donald Trump’s planned new tariffs over Greenland as “a mistake especially between long-standing allies."

Over the past year, President Donald Trump has issued multiple tariff threats against dozens of countries, only to reverse himself if financial markets slide.