Russia suspended coal supplies to neighboring Ukraine last week in a move that could leave the former Soviet republic literally in the cold this winter.

DTEK, Ukraine’s largest private energy company, said the suspension was unexpected and unexplained.

Ukraine has relied on Russian coal imports since the outbreak of a separatist uprising in the southeast of the country disrupted production there. Much of the coal produced in the region is sold to Russia instead which supports the rebels.

Coal-fired power plants normally provide around 40 percent of Ukraine’s electricity.

Just last month, Ukraine entered into an agreement with Russia for the future supply of natural gas which commits the former to prepay for deliveries.

Russia had cut off gas supplies to Ukraine when it also stepped up its support for the pro-Russian separatists fighting in the southeastern Donbas region this summer. Russia still denies it actively backs the insurrection.

The War in Donbas as well as protectionist measures enacted by Russia earlier this year in an attempt to dissuade Ukraine from signing an association agreement with the European Union have pushed its economy into recession and exhausted its foreign reserves.

The crisis was triggered when former Ukrainian president Viktor Yanukovich unexpectedly pulled out of trade talks with the European Union late last year. Hundreds of thousands of Ukrainians took to the streets to protest the decision, leading to Yanukovich’s downfall and a Russian military intervention. In March, Russia annexed Ukraine’s Crimean Peninsula which headquarters its Black See Fleet.

Ukraine’s current president, Petro Poroshenko, signed the association agreement with the European Union in June, the same month Russia cut off gas supplies.