The Ukraine crisis is forcing NATO back to the basics of collective defence in Europe as Russia seeks to restore influence in its Soviet era backyard, analysts and officials said.

NATO leaders meeting Thursday and Friday in Wales must now ensure the alliance can react quickly and credibly after years spent well beyond its original remit -- the defence of western Europe against the Soviet Union, they said.

"We now have NATO returning to it's basics, focusing primarily on collective defence and coming out of 20 years of operations with all the experience that that has given and facing new challenges that we did not expect," said Douglas Lute, the US Ambassador to the alliance in Brussels.

In the 1990s, NATO forced Serb forces out of Kosovo, to Moscow's fury at the treatment of a traditional ally.

It then took up the "Great War on Terror" after the 9/11 attacks on the United States, leading to its longest ever campaign, in Afghanistan, and intervention in Libya.

All the while, there seemed little direct threat in Europe, with Russia convulsed as it came to terms with the loss of Great Power status and helpless to prevent Soviet-era satellites joining the European Union and NATO.

Russian President Vladimir Putin has pointedly described the Soviet Union's collapse as a disaster, a calamity which must be remedied if possible.

NATO condemned his annexation of Crimea in March as a dramatic redrawing of Europe's post-Cold War borders by force and a wake-up call which the West ignores at its peril.

- Changed strategic environment -

The Ukraine crisis, NATO head Anders Fogh Rasmussen warns repeatedly, is the most serious threat to Euro-Atlantic security since the end of the Cold War.

Worse still, it comes as Russia has increased defence spending by 50 percent over the past five years while NATO countries have cut their's by some 20 percent, Ramussen said recently.

"We must reverse the trend," he said.

"NATO now faces a very changed strategic environment from just six months or a year ago," said one diplomat in Brussels, home to the alliance's HQ.

"Everyone now agrees that the alliance needs to focus on collective defence, refocusing on the fundamentals in light of the Ukraine crisis," the diplomat said.

Ukraine is not a NATO member but has been a partner country since 1997, the same year the West agreed the Founding Act with Russia which fixed Europe's post-Cold War borders and the terms for relations with Moscow.

NATO offered Ukraine full membership in 2008 but then pro-Russian president Viktor Yanukovytch dropped that option, declaring a 'non-bloc' policy in 2010.

Last week, however, Ukraine premier Arseniy Yatsenyuk said he would ask parliament to clear the way for a membership bid.

- NATO credibility at stake -

Opinions are sharply divided over whether Ukraine NATO membership would have helped avoid the current crisis by deterring Russia or made it much worse.

US President Barack Obama and other NATO leaders have insisted the alliance will stand by any member state which comes under attack, seeking especially to reassure east European members such as Poland and the Baltic states which have been badly rattled by Russia's actions in Ukraine.

In this context, the NATO summit in Wales could be a key moment, showing the alliance is still relevant and its member states ready to make the sacrifices required to face up to a more assertive Russia, analysts said.

Robin Niblett, director of the Royal Institute for International Affairs (RIIA) in London, said no one expects NATO military intervention over Ukraine but a sharp upgrade in readiness, planning and commitments will send a strong signal to Moscow.

"This needs to be a credibility summit," Niblett said.

"I think the NATO readiness action plan will have some real stuff in it," he said, referring to measures to radically speed up alliance response times.

"I think this will be a signal to Russia which it will understand.

"It may not change their behaviour but they will understand that it is real and that it carries consequences," he said.