"The risk of attack by Russia on a Nato member state, whilst still small, is significant. We are not convinced that Nato is ready for this threat," said Rory Stewart, chair of the cross-party Defence Committee.

A report by the committee urged the 28-country alliance to put permanent troops and military equipment in Baltic member nations Estonia, Latvia and Lithuania, and for headquarters to be established in the region.

It came as the European Union and the United States imposed their toughest sanctions on Russia since the Cold War, accusing Moscow of claimed failing to de-escalate conflict in eastern Ukraine.

The report described developments in Ukraine as a "wake-up call" that has revealed "alarming deficiencies in the state of Nato preparedness".

"A Russian unconventional attack, using asymmetric tactics - the latest term for this is 'ambiguous warfare' - designed to slip below Nato's response threshold, would be particularly difficult to counter," the report said.

It said tactics such as the use of cyber-attacks and irregular militias should be included under NATO's founding principle, Article 5, under which all members are bound to aid any member which is attacked.

The report called on the British government to use a September NATO summit in Wales to "lead the reordering of Nato" and drive changes to deal with a threat from Russia.

"The Nato alliance has not considered Russia as an adversary or a potential territorial threat to its member states for 20 years," said the report.

"It is now forced to do so as a result of Russia's recent actions."

A NATO spokeswoman said the report would be carefully studied, and that the alliance had already acted to up defenses, and that an "action-plan" would be discussed at the September summit.

"Nato has already taken measures to reinforce collective defense, especially for our Eastern allies, with more planes in the air, more ships at sea, and more exercises on the ground," the spokeswoman said.

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