US, Britain threaten new sanctions over Aleppo siege

The United States and Britain on Sunday threatened to impose new economic sanctions on Syria and Russia over the devastating siege of Aleppo.

US Secretary of State John Kerry branded the bombardment of civilians in the Syrian battleground city as "crimes against humanity" and British Foreign Secretary Boris Johnson urged Moscow to show mercy.

"There are a lot of measures that we're proposing including extra measures on the regime and their supporters," Johnson said, standing alongside Kerry after talks in London.

"These things will eventually come to bite the perpetrators of these crimes, and they should think about it now," he warned.

Kerry, meanwhile, warned that US President Barack Obama had not taken any option off the table in terms of tackling Syrian strongman Bashar al-Assad's assault on his own people.

He too raised the idea of sanctions but he played down the possibility of military action and insisted that it was his and Johnson's duty to "exhaust" all diplomatic options.

"We are discussing every mechanism available to us but I haven't seen a big appetite from anyone in Europe to go to war," Kerry said after talks with French and German officials.

"I don't see the parliaments of European countries ready to decare war," he said.

"Let me make it clear," added Kerry. "We are considering additional sanctions and... President Obama has not taken any option off the table."

French Foreign Minister Jean-Marc Ayrault said stopping the bombardment of Aleppo took precedence during the talks.

"We see that the regime along with Russian support has other objectives," he said following the meeting.

"We are always ready to speak with the Russians and with the Iranians but we demand that the precondition is stopping the bombardment."

Ayrault did not confirm whether a no-fly zone over Aleppo was on the table.

"We have tackled many initiatives, we have not finalised everything but there is broad consensus on the necessity for pressure (to stop the bombardments)," he said following the meeting.

Agencies contributed to this report