As the crisis in Ukraine continues to grind along and casualties continue to mount, despite the existence of a ceasefire, Russia holds a trump card.

Winter is coming and Europe will have little option but to turn to Putin for its gas purchases.

Russian President Vladimir Putin said on Friday he did not expect European countries to stop buying natural gas from Russia as there is no real alternative.

"Can you imagine that this will happen at the desire of our partners in Europe? I can hardly imagine this," Putin said in a response to a question during a meeting with Russian and foreign political experts.

Russia provides one third of the natural gas that European countries rely upon to heat their homes and drive industry. Any disruption in natural gas flows could be a potential knock against Europe's economy.

However, a long term shut off of gas would likely be unthinkable. Just as Putin can not imagine a future in which Europe could live without Russia's gas exports, so too is it difficult to imagine Russia surviving without Europe's market. Russia is simply too reliant upon energy exports to completely shut off gas to Europe for a long haul.

This reliance means that any disruption in gas to Europe would likely only be a short-term interruption meant to pressure governments. However, Europe's options are limited. Although some countries in Western Europe, such as Spain, Portugal, and Ireland, do not use any Russian gas, other nations such as Bulgaria are completely reliant upon receiving imports to heat their homes.

Map of Europe showing how heavily EU countries rely on Russian gas imports. REUTERS

The country most at risk of any potential gas shut off would be Ukraine. In 2013, Russian gas accounted for half of the gas the country used. Russia has said that it will shut off gas to supplies to the country unless it can pay over $5 billion for past expenses.

Shutting off gas to Ukraine could have a trickle down effect on the rest of Europe. In 2008, Ukraine siphoned gas from Russian pipelines passing through its territory after Russia stopped supplying the country. Putin has warned that any repeat of this action would lead to him reducing the total volume of gas he would supply to Europe.