Hillary Clinton has waded into the row over the coalition's deep defence spending cuts, warning of damage to Nato.
The US secretary of state took the unusual step of raising concerns publicly as David Cameron met military chiefs to finalise curbs after months of tough negotiations.
Downing Street moved quickly to play down the significance of the comments, stressing Mrs Clinton was referring to the wider European picture.
In an article for The Times, Defence Secretary Liam Fox also insisted that Britain would remain a "major contributor" to the international alliance even after the budget reductions.
But his Labour shadow Jim Murphy seized on the evidence of nervousness in Washington to warn the Government against "permanently damaging" the Armed Forces.
Mrs Clinton's intervention came during a visit to Brussels. Interviewed by BBC Parliament, she was asked whether she was "worried" about cuts across Europe, and particularly in the UK. "It does, and the reason it does is because I think we do have to have an alliance where there is a commitment to the common defence," she responded. "Nato has been the most successful alliance for defensive purposes in the history of the world I guess, but it has to be maintained. Now each country has to be able to make its appropriate contributions."
Mr Murphy said Mrs Clinton was right to express reservations. "We must ensure that we maintain our military effectiveness so that our partnerships and alliances in Afghanistan remain strong," he said. "There must be no cuts to defence budgets that would hamper our support for British troops on the front line."
But a Downing Street spokeswoman said: "Hillary Clinton was talking about defence cuts across Europe and specifically in the context of Nato. She is absolutely right when she says that each country has to be able to make its appropriate contribution to common defence in Nato and Britain will always do that."
In The Times' article, Dr Fox wrote: "Our recovery will have a strategic direction and we will have a global vision of Britain's place in the world. If we learnt anything from the Cold War it was that a strong economy equals strong defence. This is why I am a hawk on defence and I am a hawk with the deficit.
"There is a hard road ahead of us, but make no mistake, at the end of the process Britain will have the capabilities it needs for the future, we will continue to be a big contributor to Nato and our interests will be more secure."