Fico – who in December made a surprise visit to Moscow for talks with Russia’s President Vladimir Putin – described Kyiv’s move as “sabotage”.
The prime minister of the EU state said he would propose halting electricity exports to Ukraine and also “sharply reducing” financial support for Ukrainians who have found shelter in Slovakia.
He said there was no risk of Slovakia itself suffering from gas shortages, as it had already made alternative arrangements.
But Fico added that Ukrainian President Volodymyr Zelensky’s decision to turn off the taps would deprive Slovakia of 500m euros (£415m; $518m) in transit fees from other countries.
He said his party was ready to debate “halting supplies of electricity” and the “significant lowering of support for Ukrainian citizens in Slovakia”.
“The only alternative for a sovereign Slovakia is renewal of transit or demanding compensation mechanisms that will replace the loss in public finances,” he added.
Last month Zelensky accused Fico of helping Putin to “fund the war and weaken Ukraine”.
“Fico is dragging Slovakia into Russia’s attempts to cause more suffering for Ukrainians,” the Ukrainian president had said.
Poland has offered to support Kyiv in case Slovakia cuts off its electricity exports – supplies that are crucial to Ukraine, whose power plants come under regular attack from Russia.
Poland’s government called the cut-off “another victory” against Moscow while the European Commission said the EU had prepared for the change and most states could cope.
Moldova, which is not in the EU, is already suffering shortages.
Russia can still send gas to Hungary, Turkey and Serbia through the TurkStream pipeline across the Black Sea.