NATO Secretary-General Jens Stoltenberg said that "the whole alliance will react" if the Russian President attacks one of the 30 members of the military bloc. The former Norwegian Prime Minister's comments reflect article 5 of the original treaty that established the alliance in 1949 based upon the concept of collective self defence.

This means that an attack on one member of the alliance is judged to be an attack on all member states.

It has been invoked only once following the September 11 terrorist attacks in the United States in 2001.

In a speech in Norway on Thursday, Mr Stoltenberg said that Europe was facing its "most dangerous situation since World War 2" in a dire warning.

He also emphasised that NATO countries needed to continue to support Ukraine in order to prevent Russian aggression spreading beyond Ukraine to other countries.

Mr Stoltenberg added that this might mean that NATO countries will need to support Ukraine with weapons and other forms of assistance for a long time to come.

He said: "It's in our interest that this type of aggressive policy does not succeed.

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

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

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Following what the Kremlin calls its "special military operation" in Ukraine, previously neutral countries Finland and Sweden have applied to join NATO.

So far the request has been ratified by 23 of the 30 members of the alliance including the United States after Turkey dropped its opposition to the two countries joining.

Despite the current harsh rhetoric between Vladimir Putin and NATO, the Russian President was much more favourable towards the alliance during his first term in office during the early 2000s.

He told American filmmaker Oliver Stone during an interview that he had discussed the possibility of Moscow joining the alliance with US President Bill Clinton in 2000.

The Russian President also made similar positive comments about joining NATO during a BBC interview with David Frost the same year.

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NATO 'will react' if Putin attacks - causing 'most dangerous situation since WW2' - Express