Europe must pull together or risk being sidelined as Donald Trump signals the end of a postwar transatlantic partnership credited with keeping the peace for the past 70 years, analysts and officials say.

Fears about the US president-elect's isolationist stance became a reality this week when Trump challenged basic assumptions about the role of an "obsolete" NATO and backed the break-up of the European Union.

The shock was palpable among countries who up to now have considered themselves among Washington's closest allies but now face a sobering new reality when the billionaire businessman is inaugurated on Friday.

Europe already faces a host of problems -- Britain's impending EU exit, a nasty stand-off with Russia, and turmoil in the Middle East and North Africa driving terrorism and the worst migrant crisis since World War II.

Against this backdrop, Europe could now have to take the difficult but very costly steps required to provide security for itself after decades of sheltering under the US security umbrella.

"If EU leaders fail to pull together and speak up much more loudly than before, they risk being sidelined," said analysts Stefan Lehne and Heather Grabbe, writing for Carnegie Europe.

"To the extent that Washington engages with Europe at all, it will likely deal with countries individually. When a leader opposes his agenda, Trump's government may well seek to play off one European against another."

- Europe its own master -

European leaders reacted with shock at Trump's comments.

German Chancellor Angela Merkel, whom Trump said had made a "catastrophic decision" to let in a million refugees, insisted that "we Europeans have our fate in our own hands." Her foreign minister Frank-Walter Steinmeier said there was "astonishment" in the EU.

French President Francois Hollande meanwhile insisted Europe "has no need for outside advice to tell it what it has to do".

France and Germany have led recent efforts to create an EU defence plan, based on coordination and shared weapons procurement in order to cut costs and boost integration.

EU foreign affairs head Federica Mogherini has in turn formulated a global strategy based on the idea the bloc should achieve "strategic autonomy," coupled with increased cooperation with NATO.

NATO groups 22 of the EU's 28 member states and many of those, led by Britain but including several former Soviet satellites in the east, believe that the US-led alliance -- not Brussels -- is the only real collective defence option against a more assertive Russia.

On that basis, President Barack Obama won NATO leaders' backing for the biggest military build-up since the end of the Cold War in response to Russia's Ukraine intervention and annexation of Crimea.

But what if NATO under Trump is not so willing to come to their aid?

"If President Trump calls into question the European decision (to bolster NATO), EU member states will have to consider increasing strategic autonomy by reinforcing collective defence inside the EU," said analyst Felix Arteaga at the Elcano Royal Institute think-tank in Madrid.

A key test would be if Trump goes over Europe's heads to do a Ukraine deal with Russian President Vladimir Putin, badly wrong footing the allies, analysts said.

- Europe, US shared interests -

Other analysts say Trump may have been misunderstood or had talked too loosely.

Professor Zbigniew Lewicki, chairman of the Polish Institute of International Affairs, said: "I think he considers NATO obsolete in the sense of being ineffective fighting terrorism rather than trying to disband it, which I don't think he wants to do."

"When it comes to deeds, I don't think he will do anything to destroy NATO, or for that matter, to destroy, if he could, the European Union. I trust him more than his words."

Ian Lesser, vice president at the German Marshall Fund of the United States in Brussels, said Trump's comments should be treated seriously but not over-dramatically.

His call, for example, for the allies to share more of the NATO spending burden reflected longstanding and well-known US concerns that had also been pushed by Obama, Lesser said.

The bottom line was that the US and Europe have shared interests and values binding them together.

"The US commitment to Europe was not a bargain deal; it was driven by very important US interests and that remains the case," he said.