Russia, taking over the G8 chairmanship for the first time this month aiming to promote itself as a reliable energy source, cut its neighbour's gas supplies on Sunday.
Moscow said it had no choice but to act after Kiev refused to sign a new contract that would have raised prices fourfold, ending the preferential treatment of Soviet days.
The Kremlin describes the dispute as a commercial matter.
Kiev sees an attempt to undermine its pro-Western government and says cutting Ukrainian supplies will undermine deliveries passing through the same pipeline complex to Europe.
The move appeared to be affecting deliveries to central Europe by early evening, and Hungary and Poland reported reduced deliveries.
Washington stepped into the row, with the State Department saying it regretted Russia's move.
Sean McCormack, State Department spokesman, said in a statement: "Such an abrupt step creates insecurity in the energy sector in the region and raises serious questions about the use of energy to exert political pressure.
"The US has encouraged a compromise solution, and we remain hopeful that a resolution will be reached between the two sides that provides energy security and predictability for all concerned."
"Such an abrupt step creates insecurity in the energy sector in the region and raises serious questions about the use of energy to exert political pressure"
US State Department spokesman
Western Europe, where demand is near peak levels because of freezing weather, imports 25% of its gas from Russia, with most of that delivered by pipelines running across Ukraine.
The Russian state monopoly, Gazprom, said enough gas was being piped via Ukraine to meet its commitments to other countries.
If they were not getting all their gas, it said that meant Ukraine was tapping into it.
Hungary's gas wholesaler MOL said its Russian deliveries via Ukraine had fallen by more than 25%, forcing it to order big consumers to switch to oil where possible. Poland said supplies were down by 14%.
Germany's largest gas supplier, E.ON-Ruhrgas, warned there could be problems for big wholesale customers if the dispute dragged on.
"If the reduction in supplies should prove to be especially large or last for a long time or the winter turns out to be especially cold, then we will hit the limits of our capacities," chief executive Burckhard Bergmann said on Sunday.
German, Italian, French and Austrian energy ministers have made a joint appeal to Moscow and Kiev to keep gas flows steady, and an emergency European Union meeting is due on Wednesday.
Viktor Yushchenko, the Western-leaning president of Ukraine, is trying to take his state into the EU and Nato.
This annoys Moscow, which rejects that its influence over the former Soviet Union might be waning.
Russia wants to raise the price to
$230 per 1000 cubic metres
Ukrainian officials say that is why the Kremlin is punishing Ukraine with a huge price increase while giving ex-Soviet states that are Moscow-friendly, such as Belarus, easier terms.
Yushchenko, struggling to live up to his people's hopes after the "Orange Revolution" a year ago, says Ukraine is prepared to pay more for its gas but will not agree to a big jump at once.
Moscow wants to raise the price to $230 per 1000 cubic metres from the current $50.
Ukraine has threatened to retaliate by raising the rent that Russia's navy pays to use the Ukrainian port of Sevastopol as headquarters for its Black Sea fleet.
It also says it is entitled to skim off 15% of gas to cover transit fees.
Ukraine still has gas because of reserves and the country's own modest output, and officials say there is enough in store to see households through the winter.
But they are making no comment on the security of supplies to industry, and shortages could begin to bite within days.