Europe’s efforts to keep a united front against Russia over the war in Ukraine suffered a new blow with a surprise visit to Moscow by Robert Fico, the leader of Slovakia, for talks with President Vladimir V. Putin on natural gas supplies and the conflict in Ukraine.

Following in the footsteps of Prime Minister Viktor Orban of Hungary, who visited Moscow in July, Mr. Fico broke with the European Union’s policy of isolating Mr. Putin by meeting with the Russian president late Sunday in Russia’s capital.

In a briefing to reporters Monday in Moscow, Mr. Putin’s foreign policy adviser, Yuri Ushakov, praised Mr. Fico for his independence.

“Like Orban, Robert Fico has proved himself an independent, thoughtful politician who prioritizes the interests of his country,” Mr. Ushakov said.

The two leaders, he added, had agreed on “the importance and even necessity of restoring traditionally mutually beneficial ties between the two countries.”

Mr. Fico, a fixture of Slovak politics for decades who started out on the left and later drifted steadily to the populist right, returned to power for a third stint as prime minister after eking out a narrow win in a 2023 election. He quickly reversed the previous centrist government’s strong support for Ukraine and its hostility toward Russia.