Former Norwegian premier Jens Stoltenberg will on Wednesday take charge of a revitalised NATO which less than a year ago looked like a Cold War dinosaur in a fast-changing world.

The alliance has a newfound sense of purpose thanks to the Ukraine crisis but Stoltenberg will be aware that it must also face up to many other and longer-term challenges, analysts said.

It is simplistic to focus just on Russia, they caution, when what is needed is a "21st century threat response" which requires money and a sustained political commitment from all 28 member states.

Declining defence budgets have left "NATO forces severely over-stretched to implement deterrence against Russia in eastern Europe while confronting growing disorder in North Africa and the Middle East," said Alexandra de Hoop Scheffer of the German Marshall Fund.

As the United States turns towards Asia, Washington will expect more from its allies who will "need to define how they are willing to establish security and stability ... from Eastern Europe to Central Asia and from the Middle East to North Africa," de Hoop Scheffer told AFP.

For outgoing NATO head Anders Fogh Rasmussen, Russia's intervention in its Soviet-era satellite Ukraine was as an act of "aggression" which took the West by surprise and showed that Moscow was ready and able to use force to change international borders.

NATO accordingly could not afford not to respond to what he described as the most serious security threat since the end of the Cold War with the Soviet Union.

- New extremist threats -

At the same time, upheaval in the Middle East and Africa is spawning complex dangers which also demand an answer.

In Afghanistan meanwhile, NATO is winding up its longest-ever combat operation while remaining committed to a post-2014 training and advisory mission which it hopes will safeguard the fragile and costly gains made on the ground.

US President Barack Obama brought the strands together at a NATO summit in Newport, Wales, in early September.

He first called on the allies to back a broad international coalition to defeat Islamic State extremists in Iraq and Syria.

The summit then agreed to boost NATO readiness, setting up a fast reaction force to respond to future crises, and crucially, to increase defence spending after years of decline.

"We have reaffirmed the central mission of the alliance," Obama told his peers.

"An armed attack against one shall be considered an attack against them all. This is a binding treaty obligation. It is non-negotiable."

- Implementation difficult, costly -

Now comes the hard part -- putting those words into action.

Stoltenberg "will have to focus on implementation (of what) the alliance decided at the summit," said Jan Techau, director of the Carnegie Europe think-tank in Brussels.

The commitment to increase annual defence spending to 2.0 percent of national economic output within 10 years -- a target most members fall well short of now -- "will be very difficult: some will not abide by it, others will then be tempted to follow their example," Techau said.

"There is a political game that needs to be played very skillfully, he added.

At the same time, NATO will have to forge a working relationship with Russia given the wider dangers -- nuclear proliferation, terrorism, turmoil in the Middle East -- which both face.

NATO "should preserve the means for cooperation with Russia in the long term," which may mean "taking a pragmatic and cautious approach" over Ukraine, de Hoop Scheffer said.

But one NATO diplomat, who asked not to be named, said a tough response now over Ukraine may not necessarily be counter-productive.

"To deal with Russia successfully, you have to do so from a position of firmness, standing your ground," the diplomat said.