Just three weeks out from his retirement, United States defence secretary Robert Gates has told a European think tank there is no future for NATO under the current arrangement where the US is forced to foot 75 per cent of the bill.

He pointed out that only five members of the 28-member alliance currently spend the agreed minimum 2 per cent of gross domestic product on defence - the US, the UK, France, Greece and Albania.

On Capitol Hill, there was outrage that defence spending throughout Europe has declined by almost 15 per cent over the past decade in the aftermath of September 11.

"What I've sketched out is the real possibility for a dim, if not dismal future for the trans-atlantic alliance," Dr Gates said.

He highlighted the war in Afghanistan as a prime example of a campaign being conducted under NATO auspices but where the United States bears the overwhelming burden of resourcing and funding.

While the US maintains close to two million troops in uniform, Dr Gates says NATO struggles, at times desperately, to sustain a deployment of less than 45,000.

He added that NATO can barely provide critical support with helicopters, transport aircraft maintenance surveillance and intelligence when it comes to Libya.

The Pentagon chief said it was unacceptable that just nine of 28 NATO countries are actively engaged.

"Furthermore, the mightiest military alliance in history is only 11 weeks into an operation against a poorly armed regime in a sparsely populated country, yet many allies are beginning to run short of munitions, requiring the US, once more, to make up the difference," he said.

"Frankly, many of those allies sitting on the sidelines do so not because they do not want to participate, but simply because they cannot. The military capabilities simply aren't there."

In his final policy speech, he questioned the viability of a military alliance born in 1949 as a US-led protector against Soviet aggression.

"Future US political leaders, those for whom the Cold War was not the formative experience that it was for me, may not consider the return on America's investment at NATO worth the cost," he said.

These were sentiments voiced before but never with such vigour and from such a senior member of the administration.

So far Europe has been quiet in response. There might be a will to contribute more but in these harsh economic times, finding the way may prove more difficult.