European Union leaders lashed out at Russia Thursday, criticizing its backing for the Syrian regime and accusing it of atrocities in the besieged city of Aleppo.

"It is vital that we work together to continue to put pressure on Russia to stop its appalling atrocities, its sickening atrocities, in Syria," British Prime Minister Theresa May told reporters as she arrived for a summit of European heads of state and government in Brussels.

Estonian Prime Minister Taavi Roivas also took a hard line against Moscow and its backing for President Bashar Assad.

"They have the ambition of turning Aleppo into a new Grozny. This is absolutely unacceptable," he said, referring to the destruction of the Chechen capital in 1999-2000 by Russian troops.

The EU has imposed sanctions on Syrian officials, but unanimity is lacking within the 28-nation bloc about extending the measures to Russia.

The leaders are set to discuss their tense relations with Russia over dinner later Thursday. They had hoped to do so without the issue of sanctions weighing on their talks.

However, the siege of Aleppo and increasing civilian casualties in Syria means punitive measures are being considered once again. And as they gathered, Britain was sending warships to monitor a Russian aircraft carrier group and other vessels as they sail through the North Sea and the English Channel on their way to Syria via the Mediterranean Sea.

"I hope that we as the European Council are in a position to make clear that what is happening in Aleppo, with Russian support, is completely inhuman," German Chancellor Angela Merkel said.

"So there must be work as soon as possible on achieving a cease-fire — not just one over several hours per day, followed by many hours of bombing, but a lasting cease-fire. And above all getting humanitarian aid to people."

People in Aleppo, she said, "need a cease-fire, need an end to the war, and in their overwhelming majority have nothing in common with the terrorists."

EU Council President Donald Tusk, who is chairing the summit, said "the EU should keep all options open, including sanctions, if the crimes continue."

Swedish Prime Minister Stefan Loefven said "we probably also have witnessed war crimes, and that needs to be dealt with." On sanctions, he noted that "I don't think there is unity now ... but I think it should be on the table, that this is an option for the future."

Dutch Prime Minister Mark Rutte said it's important to keep up the sanctions pressure over Ukraine, particularly because of Russia's annexation of Crimea.

"Russia, contrary to all international rules, is still in Crimea, also in eastern Ukraine, and still causes major instability in the region," Rutte said.

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Geir Moulson in Berlin contributed to this report.