U.S. Secretary of Defense Robert delivers a speech "Reflections on the status and future of the transatlantic alliance" in Brussels, June 10, 2011.[Photo/Agencies]

BRUSSELS - In a stern rebuke, US Defense Secretary Robert Gates warned that the future of the NATO military alliance is at risk because of European penny-pinching and distaste for front-line combat. The United States won't carry the alliance as a charity case, the outgoing Pentagon chief said.

Some NATO countries bristled, but Britain quickly and heartily agreed.

Gates' assessment Friday that NATO could face "a dim if not dismal" future echoes long-standing concern of US policymakers about European defense spending. But rarely, if ever, has it been stated so directly by such a powerful American figure, widely respected in the United States and internationally.

The remarks, at the close of Gates' final overseas trip, reflect a new reality of constrained American finances and a smaller global reach.

Earlier in the week Gates played "bad cop" to US President Barack Obama's good, criticizing Germany's abstention from the air campaign in Libya two days after Obama lavished an award and fancy White House dinner on visiting Chancellor Angela Merkel.

But Gates spoke for the Obama administration, and his warning Friday was aimed squarely at Europe's priorities.

"The blunt reality is that there will be dwindling appetite and patience in the US Congress, and in the American body politic writ large, to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources or make the necessary changes to be serious and capable partners in their own defense," he said.

That assessment may cause Europeans to question the future of their defense relationship with the United States, on whom they have counted for a large measure of their security for six decades.

It comes on the heels of the withdrawal of one American combat brigade from Europe as part of a significant reduction of US troops in Europe.

The US has been the brawn behind NATO since its birth in 1949. But the disparity between strength and allies' investment has only grown wider.

In a question-and-answer session after his speech, Gates, 67, said his generation's "emotional and historical attachment" to NATO is "aging out". He noted that he is about 20 years older than Obama, his boss.

For many Americans, NATO is a vague idea tied to a bygone era, a time when the world feared a Soviet land invasion of Europe that could have escalated to nuclear war. But with the demise of the Soviet Union in 1991, NATO's reason for being came into question. It has remained intact - and even expanded from 16 members at the conclusion of the Cold War to 28 today - but European reluctance to expand defense budgets has created what amounts to a two-tier alliance: the US military at one level and the rest of NATO on a lower, almost irrelevant plane.

Gates said this presents a problem that could spell the demise of the alliance.

"What I've sketched out is the real possibility for a dim if not dismal future for the trans-Atlantic alliance," Gates said. "Such a future is possible, but not inevitable. The good news is that the members of NATO - individually and collectively - have it well within their means to halt and reverse these trends, and instead produce a very different future."

Without naming names, Gates blasted "nations apparently willing and eager for American taxpayers to assume the growing security burden left by reductions in European defense budgets".

A German foreign ministry spokeswoman defended that nation's contribution and noted Obama's recent praise.

However, defense spending is uneven within Europe.

Liam Fox, defense secretary in Britain, a strong US ally, told NATO Thursday that European governments were undermining military co-operation with the US by failing to spend enough on defense. He also said other European nations should be more willing to send their forces to NATO operations such as Afghanistan.

He praised Gates as a champion of the trans-Atlantic relationship.

"Unless Europe carries more of the share of its own defense, we should not assume his successors will do the same," Fox said.