Exports of Russian gas to Europe via Ukraine appear to have completely stopped amid a dispute over gas supplies between the two countries.
Heating systems shut down in some parts of central Europe, as outdoor temperatures plunged to -10C or lower.
Russia and Ukraine have blamed each other, and the EU says it wants its own monitors to check the flow of gas.
The EU depends on Russia for about a quarter of its total gas supplies, some 80% of which is pumped through Ukraine.
The list of countries that have reported a total halt of Russian supplies via Ukraine includes Romania, the Czech Republic, Slovakia, Bosnia-Hercegovina, Bulgaria, Croatia, Greece, Hungary, Macedonia, Serbia, and Austria.
Italy said it had received only 10% of its expected supply.
The row comes amid a cold snap across Europe that is likely to push up demand for gas.
Bulgaria says it has sufficient supplies for just a few more days.
Many other countries are now tapping strategic reserves, built up to cope with just such a development, says the BBC's central Europe correspondent Nick Thorpe.
Power stations have been told to switch to fuel oil where possible, while big industrial users have been told to prepare to limit or halt use.
Some 12,000 households in the eastern Bulgarian city of Varna had been left without central heating, authorities said. Nearby Dobrich was also affected.
In many former Soviet bloc countries whole towns and areas rely on a single centralised heating system, so that when that shuts down, every household is affected.
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