Russia's state gas monopoly Gazprom said Wednesday that Ukraine has threatened to seize gas intended for European customers if no deal is reached on gas shipments to Ukraine starting Jan. 1.

Gazprom's deputy chairman Alexander Medvedev said Ukraine's threat amounted to "blackmail" of Russia and the European Union.

He condemned the threat contained in a letter from Ukraine's state gas company Naftogaz as "utterly irresponsible," saying that violates Ukraine's obligations under previous agreements.

A seizure threat would raise fears of disruptions in supplies to Europe similar to those which occurred amid a similar dispute in January 2006. The announcement comes as Russian and Ukrainian officials remain locked in tough talks on Ukraine's debt over past gas shipments and future deliveries.

Gazprom supplies a quarter of the gas used by EU nations, and around 80 percent of it goes through Ukraine.

Ukraine's Naftogaz spokesman Valentyn Zemlyansky and other Ukrainian officials had no immediate comment, but Medvedev showed a copy of the letter from Naftogas chief Oleh Dubina at a news conference Wednesday.

Gazprom again warned it would cut gas supplies to Ukraine on Thursday and sharply raise the price for future deliveries if it fails to pay off $2.1 billion debt for Russian gas supplies by midnight.

Ukrainian gas company Naftogaz has said it paid $1.5 billion to settle its debt for Russian gas imports. Medvedev said that the sum has reached the accounts of Rosukrenergo, a Russian-Ukrainian intermediary based in Switzerland, and voiced hope it would soon reach Gazprom's accounts.

But Gazprom spokesman Sergei Kupriyanov said that it expects Ukraine to make good on the rest of its debt.