Ukraine only has half the gas it needs in storage to avoid shortages this winter, Gazprom Deputy Chief Executive Vitaly Markelov said on Tuesday.
Kiev needs 18.5bn cubic metres (bcm) of gas in storage to avoid problems, double what it currently has, Markelov said.
Moscow and Kiev are embroiled in a standoff over the price that Kiev pays Gazprom for gas. Ukraine's interim government refused to pay Gazprom after the Russian energy giant almost doubled the price it charges for gas.
"According to our colleagues (in Ukraine), 9bcm is in storage. To pass through autumn and winter periods normally, we estimate that (Ukraine) needs around 18.5 bcm (in total)," Markelov told journalists.
"So, around 9bcm more is needed," he added.
Ukraine wants to renegotiate a 2009 gas contract with Gazprom that locked Kiev into paying for a set amount of gas at the rate of $485 per 1,000 cubic metres. The price is the highest that any of Gazprom's European clients pay for gas.
Former president Viktor Yanukovych had won a reduction in the price to $268.5 per 1,000 cubic metres after he rejected an integration deal with the European Union. But he was ousted from office in February after months of massive protests.
After a pro-European interim government was installed in Kiev, Gazprom hiked the price back to the previous, more expensive rate in a move which Kiev denounced as "political" pricing and refused to pay.
Russia's deputy energy minister said on Monday it will only hold talks with Kiev over energy supplies once the Ukrainian government has paid off its outstanding gas bill to Gazprom in full.
Russian energy giant Gazprom says Ukraine owes it £3.51bn for gas deliveries and told Kiev that it must pay in advance for gas from June.
The European Union relies on Russia for around a third of its gas needs, around half of which is delivered via pipelines running through Ukraine.