Russia's state-controlled Gazprom company says Ukraine's gas bill has risen another $838 million, after Kyiv failed to pay for deliveries during the first half of June.
Gazprom CEO Aleksei Miller said on July 8 that Ukraine's overall gas debt has now reached almost $5.3 billion.
Miller said that Ukraine's "unwillingness in principle" to pay for Russian gas showed that the transition to a prepayment system was the "only correct decision."
The new bill is calculated on a price of $485 per 1,000 cubic meters that came into effect in April.
Ukraine says it will pay the debt if the price is $326 per 1,000 cubic meters.
On June 16, Gazprom cut off gas supplies to Ukraine and said that in future, Kyiv would have to pay in advance for any new supplies of Russian gas.