The British government will reveal the scale of cuts in a defense spending review next week.
The treasury has been pushing for cutting the defense budget of 37 billion pounds ($59 billion) for 2011-2015 by as much as 10 percent.
Defense Secretary Liam Fox warned Prime Minister David Cameron, in a letter leaked last month, against imposing "draconian" cuts when the country is at war in Afghanistan.
Asked if the scale of defense cuts in countries like Britain "worried" Washington, Clinton told the BBC, "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 defense. NATO has been the most successful alliance for defensive purposes in the history of the world, I guess, but it has to be maintained."
The Labor Party's shadow defense secretary, Jim Murphy, said, "We must ensure that we maintain our military effectiveness so that our partnerships and alliances in Afghanistan remain strong."