British Prime Minister Theresa May on Thursday called for a "robust and united" European approach to Russian "atrocities" in Syria, as she arrived for her first EU summit in Brussels.
"We must show a robust and united European stance in the face of Russian aggression," May told reporters.
"It is vital that we work together to continue to put pressure on Russia to stop its appalling atrocities, its sickening atrocities in Syria."
May said she brought a "clear message" to other EU leaders that Britain would continue to play its part in the bloc until it formally withdrew following June's shock Brexit referendum.
"The UK is leaving the EU but we will continue to play a full role until we leave and we'll be a strong and dependable partner after we've left," she said.
"It's in the interests of both the UK and the EU that we continue to work closely together, including at this summit."
EU leaders meeting for the two-day talks are set to "strongly condemn" Moscow and call for a permanent end to hostilities in Syria, as Russia and the Syrian army begin a brief "humanitarian pause" in the battleground city of Aleppo.
The leaders of France and Germany warned after talks with Russian President Vladimir Putin on the eve of the summit that they could not rule out sanctions over the carnage.