BERLIN (AFP) - Nato chief Anders Fogh Rasmussen said on Sunday that the Ukraine crisis underscored the need to protect the right of nations to map out their own future.

In an op-ed entitled "The right to choose", the outgoing secretary general called Russia's actions in Crimea a violation of Ukraine's territorial integrity and sovereignty, which undermined the rule of international law.

"And they flout the principle that every state is sovereign, and free to choose its own fate," Rasmussen said in an article for Germany's Welt am Sonntag newspaper which was also published in other Nato member states.

Freedom of choice helped Europe overcome its Cold War divisions, he said, but added that "the crisis in Ukraine is a reminder to us all that we must defend that principle.

"That is what Nato is doing." Rasmussen, who will step down as head of the 28-nation transatlantic alliance later this year, said Nato's door was open to new members who are ready to make the necessary reforms.

"Accession to Nato is a free choice, but it is not a free ride," he said.

Non-Nato member Ukraine's policy on joining the alliance has changed over the past decade, but cooperation between the two has "grown steadily stronger as a result", Rasmussen said.

And he referred to Nato and the European Union's expansion over the last 15 years which had provided the new members' democracies with "the strongest possible anchor" and also benefited Nato.

The right of sovereign nations to choose their own course was one of the foundations of modern Europe, he said.

"That principle has stood us in good stead since the day Nato was founded.

"We must all stand by it today." Rasmussen and US President Barack Obama, who met last week in Brussels, called for a strengthening of the military alliance on the eastern fringe of Europe, where nations once under Soviet influence fear Moscow's intervention.

But both also spoke out in favour of a diplomatic solution to the Ukraine crisis.