"Gazprom has sent today a short-notice warning threatening to reduce Russian gas transit volumes by 52 million cubic meters per day (mcm/d) in one of the major routes ... If this threat comes true this transit route to the EU would be practically excluded and out of operation," Naftogaz said in a statement issued in Kiev and Brussels.

The volume of 52 mcm/d is close to the winter consumption of a medium-sized country such as Hungary (population 10 million).

The closure of such a major artery would make it impossible for Ukraine to transfer all the gas which European Union states have paid for by other pipelines, even if Gazprom offers to supply the gas, Naftogaz said.

"This situation simply proves the fact that Gazprom is not capable to be compliant with its commitments versus the EU countries. This Gazprom decision is a direct threat to the EU's and Ukraine's energy security impacting detrimentally the whole gas transport system in Europe," the statement said.

Gas wars

Ukraine and Russia have been engaged in a war of words ever since Gazprom cut gas deliveries to Ukraine on Thursday in a row over contracts and allegedly unpaid bills.

Russia is the single largest supplier of gas to the EU, and some 80 percent of its total output flows through Ukrainian pipelines.

Each side has repeatedly accused the other of endangering the flow of Russian gas to EU markets. Both have sent diplomatic missions to Western capitals, and Ukraine has issued a series of statements to the foreign press in the EU's capital, Brussels.

Initial reports from Central and Eastern European EU states have presented conflicting views of the current rate of gas deliveries. In the latest report, on Sunday Czech officials said that the country had received 5 percent less gas than it had ordered.

The EU presidency, which the Czech Republic took over on January 1, has said it won't mediate in the "commercial dispute" between Moscow and Kiev, but insists on compliance with delivery commitments for western Europe.

EU representatives are scheduled to hold an emergency meeting on the row on Monday and could send an investigating team to the two countries within days, officials in Brussels said Saturday.