The United States sought Saturday to reassure old European allies of its continued support despite a strategic shift to Asia, amid warnings the EU could be sidelined by its economic crisis.

But as leaders, ministers and experts discussed the transatlantic alliance and Asia's rise at the annual Munich Security Conference, frantic diplomacy on the sidelines failed to prevent a veto of a UN resolution on Syria.

A top US delegation, led by Secretary of State Hillary Clinton and Defence Secretary Leon Panetta, delivered a carefully calibrated message of support to Europe, as it slashes troop numbers on the continent and looks east.

Reducing its military presence in Europe while tailoring it to future threats, Washington sees the looming strategic challenge in the Asia-Pacific as a newly powerful and assertive China rattles US allies in the region.

But Clinton stressed: "I've heard all the talk about where Europe fits into America's global outlook. I've heard some of the doubts expressed. But the reality couldn't be clearer: Europe is America's partner of first resort."

For his part, Panetta said: "Europe remains our security partner of choice for military operations and diplomacy around the world -- as we saw in Libya last year, and as we see in Afghanistan every day.

"We are therefore deeply committed to strengthening transatlantic security partnerships and institutions, including NATO," he said.

While the US military plans to withdraw two of its four army brigades stationed in Europe in 2014, Panetta announced that a US-based brigade will contribute to the NATO Response Force, a 13,000-strong unit created in 2002.

The US military will also rotate a battalion-sized task force to Germany to take part in exercises and training.

With so many top officials gathered in this southern German city, ministers engaged in frenzied diplomatic activity in an ultimately unsuccessful bid to achieve consensus for a UN resolution dealing with violence in Syria.

Both Clinton and German Foreign Minister Guido Westerwelle held bilateral meetings with Russian Foreign Minister Sergei Lavrov in an attempt to change Moscow's mind on the text under negotiation at the UN.

But Clinton acknowledged late Saturday, "I thought there might be some ways to bridge even at the last moment a few of the concerns that the Russians had.... That has not been possible."

In a swipe at Russia and China, which vetoed the text in New York, Clinton said: "To block this resolution is to bear responsibility for the horrors that are occurring on the ground in Syria."

She warned that the twin veto risked plunging Syria into "civil war."

With the eurozone crisis never far from the headlines, the conference also focused on the diplomatic implications of the turmoil -- as organiser Wolfgang Ischinger termed it, "banks not tanks."

And Europe received a stark warning from top figures such as the head of the World Bank and the Australian foreign minister that increasing insularity due to the crisis meant the EU risked missing the boat as China gains in prominence.

Australia's Kevin Rudd said as Europe becomes more "introspective" and "preoccupied with its internal problems", it is running "the risk of talking itself into an early economic and therefore globally political grave."

And Robert Zoellick from the World Bank stressed: "Frankly, most of us think the big danger is not only economic but the big insularity that has guided European policy for the last 10 years will just be exacerbated."

The prime minister of Italy, at the epicentre of the debt crisis that has threatened to tip the 17-nation bloc into recession, admitted that the turmoil has opened cracks in European unity.

The euro crisis has pitted north against south, big countries against small and has driven Britain even further to the periphery, Monti said.

"We certainly do not need in Europe to have phantoms of the past to be coming up again," said the premier.