Russia has reduced gas supplies sent to Europe via Ukraine, after allegations that Kiev is siphoning some off. Kiev has denied the claims and says technical problems are disrupting the deliveries.
Moscow stopped supplying gas to Ukraine five days ago amid a row over unpaid bills and prices.
Russian President Vladimir Putin told the head of Gazprom to reduce flow by the amount Ukraine had allegedly taken since deliveries stopped on January 1.
Countries in southern and eastern Europe have reported new falls in supplies from Russia, while Serbia and Bulgaria have urged industry to scale back demand and switch to alternative fuels.
The EU has sent a delegation for talks with both Ukraine and Gazprom officials.
A spokesman for the European Energy Commissioner said: “The information that we have received from gas companies of the countries affected indicated that, at this point in time, final consumers remain unaffected, and that there is no immediate danger for supplies to European citizens.”
A similar row between Russia and Ukraine led to shortages in several EU countries at the beginning of 2006.
The disruption comes at the worst possible time of year, as cold temperatures mean demand is high. The daytime maximum in Bulgaria yesterday was minus 5.