“I’m… here today to directly and unambiguously express that stark strategic realities prevent the United States from being the primary guarantor of security in Europe,” he told a meeting of the Ukraine Defence Contact Group at Nato headquarters.

He said the blueprint would end the “bloodshed” by bringing Ukraine and Russia to the negotiating table and backed by “robust security guarantees to ensure that the war won’t begin again”. But he warned that any diplomatic deal would have to be enforced by European armies without the help of American troops.

“The United States faces consequential threats to our homeland. We must – and we are – focusing on securing our own borders. We also face a peer competitor in China with the capability and intent to threaten our homeland and core national interests in the Indo-Pacific.”

The remarks, his first at a Nato meeting since being confirmed as Donald Trump’s Pentagon chief, will spark fears amongst European allies that they could be left on their own in the face of a resurgent Russia.