Ukrainian President Volodymyr Zelenskiy accused Slovakia’s Prime Minister Robert Fico on Saturday of opening a “second energy front” against Ukraine on the orders of Russia, as a gas transit dispute between the countries deepened.

Ukraine pumps Russian natural gas through its territory to several European countries including Slovakia, but it is expected to halt the flow when the existing transit deal - signed before Moscow’s invasion of Ukraine - expires at the end of the year.

Fico, who visited Russian President Vladimir Putin in Moscow earlier this week, said on Friday Slovakia would consider reciprocal measures against Ukraine such as halting back-up electricity supplies if Kyiv stops the gas transit from January 1.

“It appears that Putin gave Fico the order to open the second energy front against Ukraine at the expense of the Slovak people’s interests,” Zelenskiy wrote on the X social media platform.

Slovakia wants to maintain Russian gas supplies via Ukraine, saying alternative routes would hike costs and hit its own transit operations, causing it to lose 500 million euros in fees.

Ukraine has said it will not sign any new deal for gas transit with Moscow due to the invasion launched by Russia in February 2022.

Ukraine has been forced to import electricity from several of its neighbours since Russia began targeting its power grid in late 2022, damaging or destroying much of the country’s non-nuclear generation capacity.

The seized oil tanker accused of sabotaging an undersea power cable and four internet cables now sits off Finland’s coast.

Zelenskiy said Slovakia currently accounted for 19 percent of Ukraine’s power imports, and that Ukraine was working with its European Union neighbours to bolster the supply.

“Slovakia is part of the single European energy market and Fico must respect common European rules,” he wrote, adding that cutting power supplies to Ukraine would deprive Slovakia of $200 million a year.

Ukraine’s Foreign Ministry said in a statement that Fico was siding with Putin by making “senseless threats” to cut off Ukraine’s power imports.

Since taking office in 2023, Fico has been one of the EU’s most outspoken opponents of military aid to Ukraine.

After his talks in Moscow, Fico said Putin had confirmed Russia’s willingness to continue to supply gas to Slovakia, although this was “practically impossible” once the transit deal expires.