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.
Mrs Clinton's intervention came during a visit to Brussels. 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."
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.
US defence secretary Robert Gates, who is also visiting Brussels, has been voicing similar concerns about the budget pressures.
The Treasury has been pushing for a reduction of up to 10 per cent on the £37 billion Ministry of Defence (MoD) budget between 2011 and 2015.
However, Dr Fox and military top brass have argued the figure must be closer to 4 per cent if the UK's capabilities are to be maintained.
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