As the US military turns its gaze to Asia, cash-strapped Europe is under pressure to modernise its armed forces despite the economic crisis so it can police its own neighbourhood.

By declaring that the US military posture in Europe must "evolve," the new doctrine unveiled by President Barack Obama on Thursday hints that Washington will reduce its footprint on a continent it has protected for decades.

The trans-Atlantic bond, sealed over two world wars and the Cold War, is likely to remain strong, but the defence cuts and strategic shift to counter China adds urgency to calls for Europeans to beef up their forces.

"This speech is not a surprise to us," said Claude-France Arnould, the head of the European Defence Agency, which is in charge of developing ways for Europe's armed forces to pool military resources.

"The Americans are asking Europeans to cooperate more between themselves in order to develop their military capacity on a greater scale," she told AFP.

The United States has scolded Europeans for cutting military spending for years. With the eurozone debt crisis forcing governments to slash budgets, the Americans and NATO are encouraging them to spend "smarter" together.

The Libyan air war exposed the huge gap in capabilities.

While the Europeans carried out most NATO bombings, the United States provided 75 percent of all air-to-air refuelling aircraft and surveillance flights that were vital to the operation.

NATO Secretary General Anders Fogh Rasmussen has identified aerial tankers and drones as key needs but admits the cost of sophisticated military equipment is rising faster than inflation and gross domestic product.

Citing the lessons of Afghanistan and Libya, British Defence Secretary Philip Hammond this week criticised fellow NATO allies, saying the contribution of some of its members "falls short of what our collective defence requires."

"Too many countries are failing to meet their financial responsibilities to NATO, and so failing to maintain appropriate and proportionate capabilities," he said in a speech in Washington on Thursday.

"Too many are opting out of operations or contributing but a fraction of what they should be capable of," he said. "This is a European problem, not an American one."

The United States, which accounts for 75 percent of spending at NATO while the remaining quarter is shared by the 27 other allies, has warned Europeans that they cannot rely on the US military forever.

European military budgets have fallen seven percent in five years to reach 194 billion euros ($247 billion) overall in 2010. This compares to Washington's nearly $700 billion defence budget.

But the US share "is likely to drop significantly," said David Reeths, Europe director at security experts IHS Jane's A&D Consulting.

"Considering the low and dropping level of European investment in their militaries, it's hard to imagine that this would not also have a negative impact on the overall readiness and military effectiveness of NATO," he said.

The new US military strategy does not indicate how many troops may be withdrawn from Europe, but they are already way down from Cold War levels.

Some 300,000 US troops were posted across Europe at the end of the Cold War. Two decades after the Berlin Wall fell, they now number fewer than 100,000, mostly spread across Germany, Britain and Italy.

"While the report itself is intentionally vague, reading between the lines, it is clear that we must be prepared for a major reduction in US forces stationed or otherwise present in and near Europe," Reeths said.

Despite the shift to Asia, US Defence Secretary Leon Panetta said Washington was committed to NATO's Article 5, a pledge that any attack on one alliance member was an attack on all allies.

The United States still has strategic interests in Europe as it puts it closer to hotspots in the Middle East and allows it to keep an eye on Russia, said Lisa Aronsson, expert at the Royal United Services Institute in London.

"The focus on Asia doesn't necessarily mean less attention to the Middle East or Europe or Latin America," she told AFP.

"The US is a global power with global interests and there's been too much focus on South Asia and Iraq, so I think this is the right course and Europeans should welcome it."