Russia's Gazprom has confirmed it has received a prepayment of $378 million from Ukrainian state energy firm Naftogaz for gas shipments in December.
Russia's Interfax news agency on December 6 quoted Gazprom spokesman Sergei Kupriyanov saying the transfer had been made at 2:00 a.m. local time.
Naftogaz said late on December 5 that it had transferred the amount.
The move paves the way for the first shipments since Moscow cut supplies in June in a dispute over prices and debts.
Russia wants Ukraine to pay for gas in advance.
According to the terms of a deal signed in October by Naftogaz and Gazprom, gas should flow from Russia to Ukraine within 48 hours from when the Russian firm receives the transfer.
Naftogaz did not say how much gas it planned to buy, but earlier the Energy Ministry said this could be about 1 billion cubic meters.
Cash-strapped Kyiv had delayed buying new supplies, but increasingly cold weather has forced it to draw down on severely depleted reserves.
European Commission Vice President and Energy Commissioner Maros Sefcovic told RFE/RL on December 5 that the deal is only valid until the end of March, making it necessary for a so-called "summer package" to be negotiated.
"I hope can we can go over this winter without some kind of super-dramatic scenarios," Sefkovic said.
He said it was "quite clear, especially in my meetings with the Ukrainian President Poroshenko and Ukrainian Prime Minister Yatsenyuk, that we have to think very hard already in January, February and look into the ways how we can discuss the relationship in this area for the post-March period."
"I think we will do our best to again play the role of honest broker, of moderator, to make sure that I would say the same atmosphere and constructive deal is found also for the post-March period."