The reduction cuts supplies to half their normal level.
Although the Ukrainian national gas firm, Naftogaz Ukrainy, reassured EU energy officials that the dispute would not affect deliveries to Europe, a company spokesperson also said in a statement that it would "guarantee undisturbed transit to European consumers only as long as Ukraine's energy security is not
Naftogaz Ukrainy reserves the right to adopt adequate and symmetric actions to defend the interests of its customers," the company said.
The Ukrainian gas supplier operates a pipeline that provides much of the EU's gas supplies from Russia. Some 80% of Russian gas supplied to Europe comes via Ukraine.
In response, European energy commissioner Andris Piebalgs encouraged both sides to rapidly resolve their differences.
The commissioner also said he had convened a special sitting of the EU's Gas Coordination Group. The group, which brings together executives from the European gas and electricity sectors, is to plan for any possible shortages.
No member state has reported any reduction in supplies to date, said the commissioner in a statement.
Gazprom announced the reduction of supplies, saying that negotiations with Naftogaz Ukrainy over the payment of a €400 million debt were not advancing. The Russian gas monopoly also complained that no new contracts for the supply of gas this year had been signed.
However, three weeks ago, outgoing Russian President Vladimir Putin signed an agreement with Ukrainian President Victor Yushchenko assuring the payment of €980 million worth of debts to Gazprom.