Gas flow figures from Ukraine's gas transport system operator Ukrtransgaz indicate a net import of Russian gas into Ukraine for December 1, the latest available figures show.
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Normally the amount of gas Ukraine receives from Russia is more or less the same as the amount Ukraine exports through its western and southern borders.
Ukraine has not been receiving any Russian gas for its own consumption since June, only transiting gas onwards to Russia's customers elsewhere.
The imbalance reported between receipts of Russian gas and onward delivery of gas from Ukraine has been generally within 1 or 2 million cu m/day in recent weeks.
But on December 1 Ukraine received some 18.6 million cu m more gas from Russia than Ukraine exported on its borders with Poland, Slovakia, Hungary, Romania and Moldova.
The data does not reveal why flows of Russian gas into Ukraine were not matched by the same volume of flow out of Ukraine.
It could indicate more gas being consumed or put into storage within Ukraine, but there might also be some technical explanation, or a time-lag effect, with export figures possibly catching up the following day.
Ukraine is, however, planning to resume consumption of Russian gas.
The country's industry minister Yuriy Prodan Tuesday said the country is going to make an upfront payment to Russia by the end of this week to secure imports of around 1 billion cubic meters of gas.
Russia has said it will only supply Ukraine with gas if Ukraine pre-pays for it.
Meanwhile, reverse flows from Slovakia to Ukraine showed a 14 million cu m decrease on Tuesday. Slovakia had sent 28 million cu m to Ukraine in Monday, in line with recent weeks, but Slovakia's flows to Ukraine halved on Tuesday, data from Slovak operator Eustream showed.
A new metering station was being fitted that day on the Slovak side of the border, Ukraine's gas company Naftogaz Ukrainy said earlier this week.
Slovakia is nominated to send 31 million cu m to Ukraine Wednesday, latest Eustream data showed.