Hillary Clinton has admitted Washington is "worried" over the scale of the UK coalition government's planned spending cuts on defence.
The US secretary of state told the BBC that Nato must be "maintained", as it was the "most successful" defensive alliance "in the history of the world".
Mrs Clinton's comments precede next week's defence spending review, when the scale of cuts will be revealed.
Downing Street said an agreement had been "very nearly" finalised.
The Treasury has been pushing for a reduction of up to 10% on the £37bn Ministry of Defence budget between 2011 and 2015.
However, leading military figures insist the UK's defensive capabilities must be maintained.
Defence Secretary Liam Fox warned Prime Minister David Cameron, in a letter leaked last month, against imposing "draconian" cuts during a time when the UK is at war.
But Mr Cameron later said any fears over defence capabilities were "unfounded".
'Must be maintained'
Asked whether the scale of defence spending reductions in countries like the UK "worried" Washington, Mrs Clinton told BBC Parliament: "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.
"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."
Mrs Clinton also said: "We face new and different threats. Of course there are cuts that we're making but then there are new responsibilities, like cyber security or missile defence, that we're going to have to assume.
"But I have great confidence in the commitment to Nato by member nations and I believe that, despite the budgetary pressures that we all feel, we will continue to be committed to our mutual efforts."
For Labour, shadow defence secretary Jim Murphy said: "We must ensure that we maintain our military effectiveness so that our partnerships and alliances in Afghanistan remain strong.
"There must be no cuts to defence budgets that would hamper our support for British troops on the front line.
"The government must avoid rushing through enormous cuts that could permanently damage our ability to fulfil our military commitments."
Downing Street confirmed Mr Cameron had met defence chiefs for "probably" the last time before strategic defence and security review is announced next week.
The prime minister's spokesman said negotiations were "very nearly there" in finalising the plans.
A 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."