The bombardment in Syria's second city has seen hundreds of civilian deaths, the destruction of an aid convoy and an increasingly dire need for food and medical supplies for Syrians surrounded by warring factions.

"There's a lot of measures we're proposing to do with extra sanctions on the Syrian regime and their supporters, measures to bring those responsible for war crimes to the International Criminal Court," said British Foreign Secretary Boris Johnson in a joint press conference with US Secretary of State John Kerry on Sunday, referencing an upcoming EU summit on sanctions against the Kremlin.

"These things will eventually come to bite the perpetrators of these crimes and they should think about it now," he said, making it clear that Europe was not preparing direct military intervention in Syria.

Johnson also expressed skepticism that President Bashar al-Assad could retake Aleppo from rebels and "Islamic State" (IS)-related terrorists. He called on Assad's allies to promote a humanitarian truce.

"I think the most powerful weapon we have at the moment is our ability to make President Vladimir Putin and the Russians feel the consequences of what they are doing," Johnson said.

Swiss peace talks not dead

Kerry confirmed that the US was also mulling sanctions against Assad's supporters, but did not mention Russia by name.

As the crisis in Aleppo deepens, the diplomatic push to renew a broken ceasefire has seen a flurry of activity. The German Foreign Ministry reported on Sunday that days of inconclusive peace talks in Switzerland, joined by top diplomats from Russia, Turkey, Saudi Arabia and Qatar, would continue into the upcoming week.

According to the UK-based Syrian Observatory for Human Rights, while the Western diplomats were discussing their options on Sunday, 31 people were killed in airstrikes in rebel-held parts of Aleppo.

es/cmk (dpa, Reuters)