Naftogaz Ukrainy owes $600 million for gas supplied to southeastern Ukraine, Alexander Medvedev, deputy CEO of Russia's Gazprom, told reporters.

"We present general bills for gas supplied. They aren't currently taking [the bills]. Not separate bills for Luhansk and Donetsk - there's one, general bill. Gas is supplied under one contract only. There are no separate contracts to supply Luhansk and Donetsk," Medvedev said.

"If you consider one region or another to be your own territory, and then refuse to pay for the gas that the citizens of your country have used on the territory, what then do we have? Are you saying this isn't part of Ukraine? We have a single contract to supply gas to Ukraine, including the Luhansk and Donetsk regions, where Ukrainian citizens live, where Ukrainian enterprises work, where Ukrainian pensioners, teachers, doctors live," he said.