RUSSIA will attack Ukraine unless Nato gives a cast-iron guarantee it will never let Kiev join the US-led alliance, a senior Putin aide warned.

       The threat from a top Kremlin adviser is the starkest so far and stoked fresh fears an invasion could drag Western allies into World War Three.

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       President Putin's adviser issued a chilling warning of an attack on Ukraine Credit: Reuters 5

       Satellite pics show a large build-up of forces near the border Credit: AFP

       It comes after Russia massed 100,000 of troops on the border and Ukraine spy chiefs said they believe a full-scale invasion is planned in the New Year.

       And today Ukraine confirmed it fired US-made Javelin anti-tank missiles at Russia-backed rebels during fresh skirmishes in the breakaway Donbas region.

       Amid heightened tensions, Putin ally Fyodor Lukyanov warned of a “new conflict” if Nato expanded further east in an article published last night.

       Mr Lukyanov - chairman of the Russian Foreign Affairs Council, which advises the Kremlin - also made it clear that Moscow would be seeking more than verbal reassurances from Nato.

       He wrote: “This recent round of escalation in Eastern Europe showed that the old principles of security on the continent are no longer working.

       “Russia will have to change the system and draw new ‘red lines'.”

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       He made pointed reference to a post-WW2 deal between the Soviet Union and Finland, under which Moscow recognised Finland’s independence in return for its neutrality in the Cold War.

       Chillingly, Mr Lukyanov added that the “gambit that led to the 2008 war between Russia and Georgia” - when Moscow invaded after claiming to have been provoked - “could well be replicated” in Ukraine.

       Yesterday Moscow accused the US of secretly staging a mock nuclear attack 12 miles from the border "putting Russian forces on alert".

       Defence Minister Sergei Shoigu also said American strategic bombers had carried out 30 flights close to Russia this month in a significant escalation.

       Ukrainian intelligence chiefs have warned Russia is planning to invade on ten fronts in January.

       Brigadier General Budanov said the invasion would begin with air strikes and shelling following by an attack from the air involving up to 3,500 paratroopers and special forces, reports The Military Times.

       It would then be followed by a mass assault across the border, amphibious landings in Odessa and Mariupol, and a smaller invasion from neighbouring Belarus.

       Nato is 'ready at all times'

       Russia has always denied any aggressive intentions towards its neighbour - branding the latest reports as "hysteria".

       President Putin described the troop build-up as a deterrent against a renewed Ukrainian government offensive.

       Several ceasefire breaches have been reported in the eastern areas of the country held by Moscow-backed separatists.

       Kiev confirmed for the first time it has deployed the US-made Javelin anti-tank missiles in the ongoing conflict.

       General Budanov claimed they have also been used against Russian forces, who are suspected of making incursions across the border in support of the rebels.

       He said the rockets have a "significant psychological deterrent value making Russians think twice about attacking."

       Nato said it is prepared to help its allies.

       Brigadier General Simon Doran, of the US Marine Corps, said of a potential Russian invasion: “We exist to be ready at all times.”

       He told The Daily Telegraph: “Hopefully, we’re not only deterring potential adversaries, we’re also reassuring all of our partners and allies that if called upon, we will be here.

       “We are absolutely ready to combat any aggression from anybody globally.”

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       Ukraine's army has been involved in fresh skirmishes with separatist forces Credit: AFP 5

       A sniper with Moscow-backed rebels keeps a lookout in Ukraine's breakaway Donbas region Credit: Reuters 5

       Putin said the build-up of troops on the border is to deter a renewed offensive against rebels by the Ukraine government Credit: AP

       Russia warns tensions with West are ‘almost at boiling point’ and Putin ‘ready to destroy’ as Polish troops gather at the border

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