(Reuters) - Ukraine's gas transit operator said Russia had not nominated any gas flows for Jan. 1 through the Ukrainian pipeline to Europe as of 1500 GMT on Tuesday, hours ahead of the expiration of a five-year contract between Gazprom and Ukraine's Naftogaz.

Despite being at war with Russia, Ukraine still transited Russian gas to Europe through pipes on its territory under the terms of a deal signed in December 2019.

    Ukraine has repeatedly said it will not sign a new deal to replace the one expiring on Dec. 31 due to Russia's full-scale invasion of its territory, now approaching its third year.

    2025 could prove to be the first year since the collapse of the Soviet Union in 1991 when no gas will be transported through Ukraine to Europe.

    Russia used to supply a little under half of the European Union's gas before the 2022 war. But Europe has turned away from Russian gas while as yet unexplained attacks on the Nord Stream pipeline have reduced Russian supplies.

    Russia supplied a total of around 63.8 billion cubic meters (bcm) of gas to Europe by various routes in 2022, according to Gazprom data and Reuters calculations.

That volume decreased by 55.6% to 28.3 bcm last year. In 2024 transit could total less than 14 bcm of gas.

    At their peak in 2018-2019, annual flows to the European region reached between 175 bcm and 180 bcm.

    In pre-war years Ukraine received several billion dollars a year from transit, but in 2024 the payments could  drop to about $800 million due to a significant reduction in pumping volumes. 

    The Soviet-era Urengoy-Pomary-Uzhgorod pipeline brings gas from western Siberia via Sudzha in Russia's Kursk region. It then flows through Ukraine in the direction of Slovakia.

    In Slovakia, the gas pipeline is divided, with one of the branches going to the Czech Republic and the other to Austria. The main buyers of gas are Hungary, Slovakia and Austria.

(Reporting by Pavel Politiuk, additional reporting Yuliia Dysa; Editing by Gareth Jones and Louise Heavens)