Donald Trump has issued an ultimatum to Nato allies on the eve of a Nato summit, telling them to step up defence spending "immediately" or face consequences.
Washington officials have expressed concern that numerous alliance partners are falling short of their commitment to reach the five per cent of GDP threshold by 2035.
Matt Whitaker, the United States ambassador to Nato, said on Sunday evening: "But many others are lagging behind, and President Trump expects all allies to step up immediately, and not only get on a sustainable path to the 5 per cent but get to 5 per cent as soon as possible."
The warning comes ahead of the key gathering in Ankara, Turkey, which begins on Tuesday.
Britain's contributions have long been a sticking point for the US President.
Despite Britain's recently announced £15billion increase in military funding, which will bring spending to just 2.7 per cent of GDP by 2029, the figure is far below the American target.
Sir Keir has dampened expectations of finding additional funds for further rises, having already cut planned infrastructure projects to finance the current package.
He has indicated Andy Burnham, the new Labour MP for Makerfield expected to succeed him, may need to increase borrowing to cover approximately £5billion of the latest commitment.
Mr Trump's message, delivered via the US ambassador to Nato told allies to 'step up immediately' or face consequences
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The Telegraph reported this week the American President holds little confidence in Britain's capacity to resolve its defence spending difficulties.
The Turkish summit represents Sir Keir's final chance to repair relations with Washington before he steps down.
In what appears to be an attempt to appease the Trump administration, Britain has deployed its flagship aircraft carrier HMS Prince of Wales to waters off Iceland as part of Nato's Arctic Sentry mission.
The United States ambassador singled out Poland, Nordic nations and Baltic states as exemplary performers in meeting defence obligations, contrasting them with lagging members.
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Countries with higher spending are expected to receive preferential treatment in purchasing American weaponry and securing personal meetings with the President.
Secretary of War Pete Hegseth has initiated a six-month assessment of American military presence and preparedness across Europe.
This review could result in the withdrawal of US forces from nations failing to meet spending requirements.
Senior American officials emphasised on Sunday that not achieving the five per cent target would ultimately leave Europe and Canada "less capable".
They indicated Mr Trump would personally outline specific penalties for non-compliant countries at the summit.
The American President is scheduled to hold discussions with Volodymyr Zelensky during the Turkish gathering to explore ways of bringing the conflict to a close, according to a senior Washington official speaking on Sunday.
The pair previously met at the G7 gathering in Switzerland last month.
Despite Ukraine's improved battlefield position this year and recent attacks that severed power supplies to Crimea, an American official described the front lines as having "clearly frozen over the last couple of months and neither side is making a lot of progress".
The President reportedly feels a pressing need to halt the fighting.
Alliance members anticipate securing a declaration reaffirming an "ironclad commitment" to Article 5, the mutual defence clause.
National ambassadors finalised this wording at a Brussels meeting last Friday.
The summit communiqué will pledge €70billion in Ukrainian military assistance for 2026, with "at least equivalent levels" promised for the subsequent year.