The US and UK have warned the Syrian and Russian governments that new economic sanctions could be imposed if the bombing of Aleppo continues.
US Secretary of State John Kerry described the situation as "the largest of humanitarian disasters".
He said that Russia needed to understand that "this war cannot end without a political solution".
Mr Kerry is in London taking part in talks aimed at finding a peaceful solution to the Syria conflict.
Speaking alongside the UK's Foreign Secretary Boris Johnson, he said that "crimes against humanity" were taking place daily in Aleppo.
Mr Johnson also urged Russia to "do the right thing by humanity and the people of Syria" by committing to a ceasefire.
"It needs a ceasefire and it means getting back to the negotiating table in Geneva," he said.
"These things will eventually come to bite the perpetrators of these crimes and they should think about it now," Mr Johnson added.
He said that as part of measures designed to "keep the pressure up", additional sanctions on the Syrian regime "and their supporters" were under consideration.