Russia has warned that a lack of gas reserves may force Ukraine to siphon off gas in transit to European markets.
Ukraine's gas reserves are in a "critical" condition, according to Russia's Energy Minister Alexander Novak.
Russia's state energy giant Gazprom cut off gas deliveries to Ukraine in June after a price dispute between Moscow and Kiev.
Ukraine refused to pay an inflated price for gas after Russia annexed the Black Sea peninsula of Crimea in March.
While it has stored some reserves and has experimented with reversing gas flows from some EU countries, Ukraine's government has admitted it may need extra supplies to last the winter.
Novak said Ukraine needs to store more gas underground, around 10bn cubic metres, in order avoid running out of supplies over the winter.
Moscow has accused Ukraine of siphoning off gas in transit to European customers during prior disputes over pricing in 2006 and 2009.
The EU relies on Russia for about a third of its gas needs, half of which reaches its markets via Ukrainian transit pipelines.
The European Union has moved in recent weeks to shore up its gas supplies for the winter as the conflict between Russia and Ukraine shows no sign of slowing down.
The EU's Energy Commissioner Guenther Oettinger said Moscow and Kiev must work harder to resolve the gas price dispute and agree on an interim price to prevent Ukraine running out of gas.
The EU imposed its toughest economic sanctions against sectors of the Russian economy in late July. While oil companies were targeted, along with banks and the defence sector, the gas industry was left out of the sanctions.