London: UK Prime Minister Keir Starmer said a "trade war is in no one's interest" on Monday, after US President Donald Trump vowed to hit Britain and European countries with tariffs over their support for Greenland.

"The use of tariffs against allies is completely wrong," Starmer told a hastily-arranged press conference after Trump threatened to slap tariffs on Britain, Denmark -- of which Greenland is an autonomous territory -- and other European countries unless the Arctic island is ceded to him.

"Alliances endure because they're built on respect and partnership, not pressure," Starmer said, adding he urged Trump in a call on Sunday to find a solution "rooted in partnership, facts and mutual respect".

"We will keep dialogue open. We will defend international law and we will use the full strength of government at home and abroad, to protect the security living standards and future of the British people."

"We've worked very closely with the United States, and we must never lose sight of our national interest in that," he added.

The prime minister acknowledged it was a "serious" situation, but said that it would not push him to "choose between the US and Europe", as he walks an increasingly shaky tightrope between Britain's two closest allies.

Germany's vice-chancellor on Monday said Europe was preparing countermeasures to Trump's "blackmail".

Starmer said there was "proper unity and coordination" in the response from the UK and EU, but the UK was not "at the stage" of thinking about imposing retaliatory tariffs. (AFP)