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.
See map of affected area
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.
Venting anger
Russia and Ukraine have been blaming each other for the disruption to Europe's energy supplies.
Dependence on Russia for gas:
Russia's gas company Gazprom has accused Ukraine of shutting off the final pipeline carrying gas to Europe, but Ukraine's Naftogaz said that would be impossible, since the taps are in Russia.
Correspondents say the differing versions offered by the two countries show how far apart they are, and that the row is rapidly becoming a means for venting anger caused by poor political relations.
European Commission President Jose Manuel Barroso accused the two countries of taking the EU's gas supply "hostage" and urged them to resume supplies immediately.
He said the EU was ready to send monitors to the Russian-Ukrainian border, and at Ukrainian exit points, to determine where the shortfalls originated.
Russia has called for such monitors and Ukraine's prime minister said they could start work "immediately", according to the AFP news agency.
Talks between Naftogaz and Gazprom aimed at resolving the crisis are due to resume in Moscow on Thursday - after the Christmas public holiday on Wednesday in Russia and Ukraine.
Gazprom will also discuss the matter with the EU on the same day.
Back to top
Have you been affected? Are you worried by the crisis?
Send your comments using the post form below.
In most cases a selection of your comments will be published, displaying your name and location unless you state otherwise in the box below.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©
~RS~q~RS~~RS~z~RS~56~RS~)