(Reuters) - Ukrainian President Volodymyr Zelenskiy on Monday criticised what he said was Slovak Prime Minister Robert Fico's lack of desire to end his country's dependency on Russian natural resources as a "big security issue" for Europe and Slovakia.

Zelenskiy made the comments after Fico met with Russian President Vladimir Putin in Moscow on Sunday. Fico said the meeting was in response to talks with Zelenskiy, who he accused of opposing any gas transit through Ukraine to his country.

"His key goal is to deal with Russia, and this is what benefits him. This is indeed a big security issue - both for Slovakia and the entire Europe," Zelenskiy said on X.

"Why is this leader so dependent on Moscow? What is being paid to him, and what does he pay with?" he added.

Ukraine has repeatedly said it would not extend a transit deal for gas flows from Russia to Europe, which expires on Jan. 1. Payments for the gas have helped Russia to wage its almost three-year war in Ukraine, Kyiv says.

The flow through Ukraine accounts for around half of Russia's total pipeline gas exports to Europe. Slovakia, Italy, Austria and the Czech Republic will be the most affected if it ends.

Slovakia, which has a long-term contract with Russia's Gazprom, has been trying to keep receiving gas through Ukraine, saying buying elsewhere would cost it 220 million euros ($229 million) more in transit expenses.

Zelenskiy said that Moscow was providing significant discounts for Fico but added that Slovakia was paying for them by its sovereignty or via "murky schemes". He gave no details.

"This should be a matter of concern for Slovakia's law enforcement and special services," he added.

Slovakia's government office could not be immediately reached for comment and did not immediately reply to emailed questions.

The Ukrainian leader said that the issue was raised at a meeting of European leaders in Brussels last week. Fico has refused compensation to ease the transition period and get rid of dependence on Russian gas, Zelenskiy added.

"(Fico) wants to assist Russia in pushing American gas and energy resources of other partners away from Europe, implying that he wants to help Putin earn money to fund the war and weaken Europe," Zelenskiy added.

Fico said on Sunday that Putin had confirmed Russia's willingness to continue to supply gas to Slovakia. Kremlin spokesman Dmitry Peskov said he could not give more details.

(Reporting by Yuliia Dysa; Editing by Andrew Cawthorne and Sharon Singleton)