NATO has warned Europe is too reliant on Russian gas and must diversify its supply to avoid being cut off amid the Ukrainian crisis.

Secretary-General Jens Stoltenburg said the organisation was "concerned about the energy situation" amid fears President Vladmir Putin could stop gas supplies to Europe if sanctions are imposed if his troops invade Ukraine.

Europe depends on Russia for around a third of its gas supplies and any interruption would exacerbate an existing energy crisis caused by a shortage.

Russia has massed some 120,000 troops near its neighbour and demanded the western defence alliance pull back troops and weapons from eastern Europe and bar Ukraine, a former Soviet state, from ever joining Nato.

There are major fears Russia is going to invade Ukraine - a move that would lead Nato to imposing economic sanctions on Russia.

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Asked on the BBC this morning if he was concerned Russia could cut off gas supplies to Europe in retaliation, Stoltenburg said: "We are concerned about the energy situation in Europe because it demonstrates the vulnerability of being too dependent on one supplier of natural gas and that’s the reason why Nato allies agree that we need to work and focus on diversification of supplies."

He added: "We also look very carefully at what the International Energy Agency said recently that Russia has manipulated the European gas market by holding back supplies, by depleting the storage facilities of natural gas and this highlights the importance of developing alternative sources for energy supplies to Europe because what we face now is serious with very high prices for many European customers."

Stoltenburg went on to say that Nato allies were trying to get alternative supplies of liquid natural gas from other parts of the world, including the United States and Qatar.

He added: "Then in the more longer term this is about reducing the dependency on these types of energy sources to develop new and also environmentally friendly domestic energy sources but that will take some more time and require big investments by European allies."

His warning about European countries being too reliant on Russian gas comes just months after First Minister Nicola Sturgeon signalled an end to "unlimited fossil fuel extraction" in a bid to reduce CO2 emissions.

She later said the proposed Cambo oil field off Shetland "should not get the green light". The decision on whether drilling should be allowed sits with UK authorities.

On Sunday night the industry's representative OGUK Energy Policy Manager Will Webster said North Sea gas could help alleviate supply pressures.

“If the Russians reduce deliveries of gas to Europe, then it has to come from somewhere else, most likely as shipments of liquefied natural gas.

"That will increase competition for supplies, driving up prices and consumer bills even more. Conversely, any additional gas we produce ourselves will help alleviate this process," he said.

“In the longer term, if UK gas production is allowed to fall as predicted, then our energy supplies will become ever more vulnerable to global events over which we have no control – as we now see happening with Russia’s threatened invasion of Ukraine.”

Earlier in the interview Stoltenberg said there is “no certainty about the intentions” of Russia following rising fears that an attack on Ukraine is imminent, but added there is a “real risk”.

He told BBC Sunday Morning: “What we see is a very significant Russian military build-up with more troops or more heavy equipment, but also threatening rhetoric.

“Then we have the track record of Russia using force against Ukraine before, but there is no certainty about the intentions.

"Of course, what Nato allies do is that we send a clear message to Russia that they should not use force against the Ukraine once again, and we’re also pursuing a diplomatic track to prevent them from doing so because it is in no-one’s interest to have armed conflict in Europe.”

“There is a real risk and that’s exactly why the Nato allies have stepped up with more support to Ukraine,” he added, warning that if Russia uses any force again on Ukraine there will be a “high price for them to pay with economic and political sanctions”.

US officials said on Saturday Russia’s military buildup had been expanded to include supplies to treat casualties of any conflict. Across the border in Ukraine, locals trained as army reservists as the government scrambled to prepare.

Moscow denies any plan to invade but said on Sunday it would ask Nato to clarify whether it intends to implement key security commitments after earlier saying the alliance’s response to its demands did not go far enough.

“If they do not intend to do so, then they should explain why,” Russian Foreign Minister Sergei Lavrov said on state television. “This will be a key question in determining our future proposals.”

The United States has said it is waiting to hear back from Russia. It says Nato will not withdraw from eastern Europe or bar Ukraine but it is prepared to discuss topics such as arms control and confidence-building measures.