RUSSIA’S most likely invasion plans have been revealed to The Sun as tensions soar over Ukraine.

       Vladimir Putin could trigger a D-Day style landing to capture the port of Odessa, a lightning strike against Kiev or a two-pronged pincer move from western Russia and Crimea to annihilate Ukraine’s forces east of the Dnieper River.

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       A Russian serviceman taking part in a military drill as fears of invasion escalate 6

       Russian S-400 Triumf surface-to-air missile systems lined up during military drills 6

       It comes amid British warnings of significant numbers of Russian advance forces already inside Ukraine – including Spetznaz special forces and spies.

       They are building up lists of likely collaborators to help govern the towns they might conquer.

       Up to 127,000 Russian troops have surrounded Ukraine on three sides to stretch the defending forces and keep them guessing about Moscow’s next move.

       And fleet of six Russian landing ships is steaming through the Mediterranean to add pressure on Ukraine’s Black Sea coast.

       But the force is still only half what is needed to conquer and occupy all of Ukraine, according to western officials.

       “America took a force of 250,000 troops to invade Iraq in 2003,” a source said. “It is not all about numbers. It is about capabilities.

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       "Ukraine is the biggest country in Europe, bigger than Britain and Germany combined. You would expect a similar sized force, at least, to try to take and hold Ukraine.”

       Putin could muster the extra troops in as little as two weeks, an intelligence source said.

       But the size of the current build up is fuelling speculation that the Kremlin is planning what Joe Biden described as a “minor incursion”.

       That could include a mission to seize and rebuild the Crimea canal.

       It used to be a lifeline, bringing water Dnieper River, but the Ukrainians turned off the taps after Russia seized the peninsular in 2014.

       “If they want to launch an attack that is smaller and shorter than a full-scale occupation, they could do so today with little or no warning,” the top western spook told The Sun.

       Other scenarios include a lightning strike on Kiev.

       British military estimates suggest 35,000 Russian troops are already inside Belarus just a few hours drive from the Ukrainian capital.

       The troops are due to start war games next week, with tanks, jets and air defence systems, which would offer them perfect cover for an invasion.

       Boris Johnson warned that Putin had enough troops to launch a “lightning war” against Kiev, but he said it would be “painful, violent and bloody”.

       "If Russia pursues this path, many Russian mothers’ sons will not be coming home,” he said.

       A third scenario involves seizing Odessa, Ukraine’s third largest city, a tourist destination and a major Black Sea port.

       In that case troops would swing west from Crimea while Ropucha-class landing ships would launch a “simultaneous amphibious assault”.

       TWO-PRONGED ATTACK

       One of the worst scenarios presented to Western leaders involved a two-pronged attack from the south and east.

       A Russian column would surge north from Crimea while their comrades in the motherland push west through occupied Donbas.

       That could leave Ukraine’s armed forces facing “a double envelopment” trapped between two Russian lines.

       “Best case scenario is I'm just boxed in and I can't leave,” the western spook said.

       “Worst case scenario is that either lose all my forces because they're captured. Or worst case, I lose them all because they're annihilated.”

       Officials said Putin’s “Plan A” may still be to win without fighting. But they can’t be sure what what Russian president wants.

       Victory for him could mean toppling Ukraine’s west-leaning government and installing a puppet regime.

       It could be concessions from Nato – which western officials have rejected – over banning Ukraine from joining and reducing the number troops an exercises in eastern European countries.

       Or, as many fear, he may be set on trying to recreate the Soviet Union, to reverse what he sees as decades of Russian humiliation since the end of the Cold War.

       A top western spook said the Kremlin’s strategy was like holding a gun to their victim’s head.

       “If I meet you in an alleyway and I put a gun in your face and I say, ‘Give me your wallet, or I'll kill you,’ can I achieve my objective of receiving your wallet without actually shooting you? Yeah, I probably can And I think the Russians can also I think they think they can,” he said.

       'CRUNCH TIME'

       The window for launching an invasion is closing.

       Putin will have to make a decision soon, a senior Nato source said.

       The ground in Ukraine is expected to thaw from mid-March meaning tanks get bogged down in the mud.

       "If it's not before March 17, that's probably it for another year," the source said.

       The upcoming Beijing Winter Olympics could also sway Putin's decision.

       He invaded Georgia in 2008 two days after the openning ceremony of the Beijing Summer Games.

       If he invades Ukraine during these winter games, from 4 to 20 February, he risks upsetting Bejing - although Russia and China have both denied claims that they discussed any dates for the conflict.

       "There is a three week window between the end of the games and start of the thaw," the source said.

       "That's crunch time."

       Defence Secretary Ben Wallace is due to fly to Moscow in February to meet his Russian counterpart as part of emergency talks to avoid a war.

       Asked if he thought war was inevitable, he told The Sun "not yet".

       But the longer the Russian troops stay in the field the path to peace gets harder.

       “The scale of the build-up is so large, that it's hard to imagine…a peaceful outcome,” the western spook said.

       “The longer it continues to grow and stay in the field, some conflict on some scale seems to me to be unavoidable.”

       It comes after Joe Biden reportedly warned Ukraine's president to "prepare for impact" as Russia could invade in days.

       It's understood the US president told Volodymyr Zelensky that a February invasion is now "virtually certain" with more than Russian 120,000 troops massed on the border within striking distance.

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       Ukraine has been holding military exercises as it fears invasion from Russia 6

       A Ukrainian service member holds a next generation light anti-tank weapon, supplied by Britain, during drills 6

       Several combat ships and vessels of the Russian Black Sea Fleet took part in drills this week