NATO chief: Ukraine war is Europe's most dangerous time since WW2

The war in Ukraine is the most dangerous moment for Europe since World War Two, and Russia must not be allowed to win, NATO Secretary-General Jens Stoltenberg said on Thursday.

To prevent Moscow from succeeding, NATO and its member countries may have to continue to support Ukraine with arms and other assistance for a long time to come, he said.

"It's in our interest that this type of aggressive policy does not succeed," the former prime minister of Norway said in a speech in his native country.

"What happens in Ukraine is terrible but it would be much worse if there was a war between Russia and NATO," he said.

Describing what Moscow calls a "special military operation" as an attack on the current world order, Stoltenberg said the alliance had to prevent the war from spreading.

"This is the most dangerous situation in Europe since World War Two."

"If President (Vladimir) Putin even thinks of doing something similar to a NATO country as he has done to Georgia, Moldova or Ukraine, then all of NATO will be involved immediately," Stoltenberg said.

The war has led previously non-aligned Finland and Sweden to seek NATO membership, with the request so far ratified by 23 of the 30 member states, including the United States.

"This is not just an attack on Ukraine, an independent democratic nation with more than 40 million people, it's also an attack on our values and the world order we want," the NATO chief said of the war.

Stoltenberg spoke at Utoeya island where a far-right extremist in 2011 killed 69 people in a shooting spree targeting members of the governing Labour Party's youth wing. In the years since the attack, the small Norwegian island has become a centre for teaching democratic values.