Britain repeated its call on Tuesday that President Bashar al-Assad should step aside, citing his "barbaric cruelty" toward Syria's people, as his forces stood poised to recapture Aleppo.
"We do not think that President Assad, who is presiding over such barbaric cruelty to the people of Syria, is a route to a long-term, secure and prosperous future for Syria," Prime Minister Theresa May's official spokeswoman told reporters.
"That's why we think there needs to be a political transition away from Assad."
The UN said Tuesday it had credible reports of pro-government forces in Aleppo executing dozens of civilians including women and children as the crucial battle for the city neared its end.
Ms May's spokeswoman said the reports coming out of the city were "extremely concerning".
"Our first priority has absolutely got to be to get aid to those whose lives are in danger," she said.
"More broadly we continue to work with partners to look at what steps we can take to make sure people are held to account for their actions there, and that work will continue."
Britain will press fellow European Union leaders at a summit on Thursday to secure a "strong, clear statement" on the situation in Aleppo, "the need for humanitarian access and for a ceasefire", she said.
Mr Assad's forces look set to overrun the last pocket of rebel territory in east Aleppo, dealing the biggest blow to opposition fighters in more than five years of civil war.
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