London, Sep 7 : Vladimir Putin's 'energy blackmail' over Europe could lead to 'civil unrest' this winter, the NATO Secretary General has warned, Daily Mail reported. Jens Stoltenberg acknowledged that winter 'will be hard' as 'families and businesses feel the crunch of soaring energy prices and costs of living' in the coming months. Writing in the Financial Times, the boss of the Western security alliance said that it is worth paying the price to support Ukraine. He said: 'Winter is coming and it will be hard. Hard for the Ukrainian people and armed forces who are fighting for their freedom, and hard for those of us who support them. 'Our unity and solidarity will be seriously tested, as families and businesses feel the crunch of soaring energy prices and costs of living caused by Russia's brutal invasion. "We face a difficult six months, with the threat of energy cuts, disruptions and perhaps even civil unrest. But we must stay the course and stand up to tyranny - for Ukraine's sake and for ours." Stoltenberg warned that Putin could carry out further aggression against Russia's neighbours and even attack NATO allies if he is emboldened by success in Ukraine. Today, Putin denied that Moscow was using its energy exports as a 'weapon' after Russia halted natural gas deliveries via a key pipeline to Europe, Daily Mail reported. The Nord Stream pipeline has been closed for several days after Russia said that it would be under repair for an indefinite period of time, reinforcing fears of an energy price crisis in Europe with winter approaching. /IANS