Russian gas giant Gazprom said it will halve gas supplies to Ukraine today because a debt dispute between them has not been resolved.
Yesterday, the world's top gas producer and Russia's gas export monopoly reduced supplies to Ukraine by 25 per cent over a $600 million debt and a conflict over deliveries in 2008.
Ukraine will get around 70mcm of gas per day, or 50 per cent of the usual volumes from 5pm today.
Both Gazprom, whose chairman Dmitry Medvedev has just been elected Russian president, and Ukraine said supplies to Europe were running normally.
Almost a quarter of Europe's gas supplies pass through Ukraine, and gas disputes between Moscow and Kiev tend to cause anxiety in European energy markets.
Though all of Gazprom's exports are reaching Europe, Ukraine said the firm had not paid transit fees for gas going to Europe since December, a claim denied by Gazprom.
Gazprom has threatened to cut gas supplies during previous pricing disputes with former Soviet neighbours Ukraine and Belarus.