Central European countries rely heavily on Russia for gas.
Western Europe gets 25% of its gas from Russia - 80% of it via Ukraine.
Most of it is destined for Germany with some also going to France, Italy and Austria.
Some of Russia's European gas customers are already feeling
the impact of reduced supplies.
The following is a fact file on how they are coping:
Hungary reported gas supplies down by 40%. Russian gas amounts to about 80% of Hungary's needs.
MOL, the country's national wholesaler, said households would not be immediately threatened but joined providers in other nations in urging major industrial consumers to switch to back-up oil systems.
And Sandor Kantor, an MOL official, said his company was cutting deliveries southward to Serbia and Bosnia-Herzegovina.
Serbia said it was receiving 50% less gas than normal. Moldova has not received Russian gas supplies for two days, Vladimir Voronin, the president, said, adding that prices had inexplicably doubled, making it too expensive for the former Soviet republic to renew a contract with Russian company Gazprom.
Piotr Wozniak, Poland's economy minister, said gas deliveries at the Polish-Ukrainian border had risen to about 65% of the daily norm, from a low of 50% on Sunday.
Romania's deliveries from Russia were down by 25%, said officials, but unseasonably warm weather eased immediate concerns.
In Austria, the OMV energy conglomerate said that natural gas imports from Russia via Ukraine were down by a third.
Slovak state oil company official Alexander Nemudrov told Russia's Gazprom-owned NTV television on Monday that it was receiving just 60% of the gas it needs to export further West.
German gas companies EON Ruhrgas and Wingas said on Monday they were experiencing disruptions in the supply of gas from Russia.
"We are clearly receiving less than foreseen in our contract but we cannot yet specify how much," an EON Ruhrgas spokeswoman said, as Wingas also reported irregularities in its supply from Russia.
European countries are suffering
from reduced gas supplies
The spokeswoman said the company had been able to compensate for the reduction in supply "without a problem" but added that if the disruption continued, EON Ruhrgas would increase its imports from other countries.
German industry can draw on stocks holding up to 75 days worth of gas as well as imports from other countries such as Norway and the Netherlands, an economy ministry spokeswoman said.
Italy and other nations with diversified gas sources were less affected.
But Italian oil and gas giant Eni Spa on Monday was measuring a "considerable reduction" in the pressure of the gas flow from Russia, Italian news agencies reported.
Czech supplies from Russia via Ukraine were so far unaffected, importer RWE Transgas said.
The Czechs receive about two-thirds of their gas from Russia, the rest from Norway.
France, through French utility Gaz de France, said it was following the situation very closely and looking at precautionary measures it could take to secure supplies.