Greek Prime Minister Alexis Tsipras holds a news conference Thursday with European Parliament President Martin Schulz in Athens. (Milos Bicanski/Getty Images)

Just days after shaking European economic policy to its core with a sweeping win in Greek elections, the radical leftist party Syriza is challenging a fundamental tenet of the continent’s foreign policy by seeking a softer stance on Russia.

Both before and after coming to power this week, party leaders have made no secret of their affinity for the Kremlin. They visited Moscow to show solidarity after Western condemnation of the Russian annexation of Crimea last spring. New Greek Prime Minister Alexis Tsipras made the Russian ambassador his first foreign visitor within hours of taking office Monday.

Now Syriza is complicating Western efforts to take a tough line against Moscow amid an escalating Russian-backed insurgency in southeastern Ukraine.

The new dynamic was on display Thursday, with European foreign ministers gathered for an emergency meeting in Brussels to consider fresh sanctions against Moscow just days after shelling killed 30 civilians in the Ukrainian port city of Mariupol. But amid Greece’s doubts, the ministers could agree only to extend existing sanctions while deferring any decision on new ones after hours of emotional debate.

“The discussion was open, frank and heated,” Lithuanian Foreign Minister Linas Linkevicius said in an interview.

Although Greece is just one of 28 members of both the European Union and NATO, both organizations operate on a principle of unanimous consent, meaning any member can block policy with a simple veto.

After years of Russian support for populists on the far right and far left in an attempt to undermine European unity, the election of Syriza gives Moscow a potentially critical spoiler at the heart of Western decision-making.

“You have a lot of people asking themselves whether Greece is going to play the role of the Trojan horse,” said Ben Nimmo, a European security analyst and former NATO official. “But nobody really knows. And you have mixed messages coming out of the Greek government.”

Still, the prospect of a Russian beachhead inside Western alliances has stirred Cold War-style fears within European defense ministries this week.

“If you can’t sit down in a NATO meeting in Brussels, dive into the intelligence and be sure that it’s not going straight back to the Kremlin, that’s a pretty significant and shocking development for the alliance,” said Ian Kearns, director of the European Leadership Network, a London-based think tank.

But just how far Greece’s new government will go in hewing to a pro-Russian line remains unclear.

At Thursday’s foreign ministers meeting, Linkevicius said, he was heartened that there was no veto of existing sanctions. Although expanded measures were kept on the table, he acknowledged that enacting them “will not be easy.”

The threat of disruption to Europe’s Russia policy, some officials said, may be a mere tactic ahead of broader and, for Greece, more important negotiations to come over the terms of the country’s mammoth debt. Syriza has demanded that the country’s $284 billion bailout agreements be renegotiated, with a significant portion of the total forgiven and austerity restrictions lifted.

“This is a way of finding leverage for the future,” said Artur Habant, spokesman for Poland’s E.U. representative. “They have a lot of issues to discuss with the E.U. And their survival, frankly, is much more important for them than the question of Ukraine.”

Before the party’s victory Sunday, Syriza’s leadership was outspoken in defending Russia against Western criticism. Last spring, Tsipras visited Moscow and met with Kremlin associates. Western sanctions, he said, were counterproductive.

“I’m sure the E.U. should conduct dialogue and seek peaceful ways out of the conflict together with Moscow and not impose sanctions on Russia,” Tsipras told the state-owned daily newspaper Rossiiskaya Gazeta.

Since Sunday, Syriza has doubled down in its backing for Russia. Tsipras had been in office for only hours Monday when he welcomed his first foreign visitor, the Russian ambassador. The second was the Chinese ambassador.

Greece objected vehemently when European Council President Donald Tusk on Tuesday issued a statement condemning Moscow for the shelling of Mariupol and asking European foreign ministers to draw up new sanctions. The Greek government, Athens said, had never been consulted.

For Syriza, challenging the E.U. stance on Russia reflects an ideology “that says we have to be skeptical of certain things our European partners do because the E.U. is a capitalist, neoliberal enterprise,” said Spyros Economides, an international relations professor at the London School of Economics.

For Russia, he said, support for Syriza is more “a marriage of convenience.”

Some Russian officials have responded to Syriza’s triumph with undisguised glee.

“Syriza's victory will be a breakthrough and will destroy Europe’s liberal consensus,” Mikhail Emelyanov, head of the Russian Duma’s committee on economic policy, told the state news service RIA Novosti.

Russia has taken pains to win over populist European parties with cash and charm offensives. Last year, French right-wing leader and E.U. critic Marine Le Pen acknowledged that her party, the National Front, had taken an $11 million loan from a Russian bank. Le Pen has spoken warmly of Putin.

If Greece does dissent from the European consensus and take a pro-Moscow stance, it could be a return to form for a country whose socialist governments occasionally leaned in that direction at the height of the Cold War.

Greece is hardly the only country in Europe that has had misgivings about sanctioning Russia since the war in Ukraine began last year. A variety of countries — including Germany, Greece’s nemesis in prospective debt talks — have at various times resisted coming down hard on Moscow, given the deep ties between the Russian and European economies.

Now, the inclusion of an openly pro-Russian government in the E.U. may give other nations cover for their own dovish stances.

“Some of them will be seeing this as an opportunity to hide behind the Greeks,” Kearns said.

Demirjian reported from Kiev, Ukraine. Karla Adam in London contributed to this report.