US concern over Europe's military spending
Hillary Clinton's admission that Washington is "worried" over the scale of the UK coalition government's planned spending cuts on defence comes ahead of next week's defence spending review.
She also said Nato was the "most successful" defensive alliance "in the history of the world" and must be "maintained".
BBC defence correspondent Caroline Wyatt examines the source of Mrs Clinton's concerns over Europe's military might.
It is unusual for a US Secretary of State to express such concerns so openly. But her remarks were not addressed to the UK alone.
Washington has long made clear that in terms of military spending, its European Nato partners must try harder.
Nato struggles over the past few years to persuade some of its NATO allies in Europe to provide sufficient forces willing to face combat in Afghanistan with few caveats, and supply crucial equipment such as helicopters, led to bitter jokes amongst US troops.
They would say the acronym Isaf (International Security and Assistance Force) actually stood for "I Saw Americans Fight".
And that was before the recession.
Now, the impact of the economic crisis across Europe is likely make itself felt in defence spending, especially in the UK, which has traditionally been one of the largest contributors to NATO missions abroad, second only to the US.
It is one of the few Alliance members in Europe to spend the agreed 2% or more of its GDP on defence.
The UK has also been willing to send troops to fight in some of the toughest areas in Iraq and Afghanistan over the past decade, with its special forces and intelligence capabilities considered particularly crucial by Washington.
The US Secretary of State's comments have been seen as interference by some.
However, with the UK defence secretary Liam Fox still fighting his corner against the Treasury as he seeks the best possible financial settlement for the MoD's budget over the coming years, Hillary Clinton's words may not have been an unwelcome intervention.
They underlined that the decisions the UK takes on defence in the coming days are not purely internal or domestic, but matter enormously to Britain's closest international allies as well.
This, at a time those allies are also looking to their own future spending levels, and perhaps hoping their NATO partners will step in to shoulder more of the costs of Nato future campaigns.