THE UK and the US are considering implementing economic sanctions against Syria and Russia in response to the bombardment of Aleppo.
Foreign Secretary Boris Johnson warned Syrian president Bashar al Assad and the Russians that they would not be successful in their "barbaric siege" of Aleppo, but said the Russians can be made to "feel the consequences of what they are doing".
Speaking following talks in London on the war in Syria, US Secretary of State John Kerry said the bombardment constituted "crimes against humanity".
He said: "It could stop tomorrow morning, tonight if Russia and the Assad regime were to behave according to any norm or any standard of decency, but they've chosen not to.
"Instead we see what can only be described as crimes against humanity taking place on a daily basis, and hospitals are bombed and children are bombed or gassed."
More than 275,000 civilians in the city are enduring daily bombing raids, alleged gas attacks, and the use of highly destructive barrel bombs.
The move towards imposing sanctions follows speculation over British and US military involvement in the country.
However, Mr Johnson said there remained a "lack of appetite" in Europe for such intervention.
He said: "We are pursuing diplomacy because those are the tools that we have and we're trying to find a way forward under those circumstances.
"No option is in principle off the table but be in no doubt that these so-called military options are extremely difficult and there is, to put it mildly, a lack of political appetite in most European capitals and certainly in the West for that kind of solution at present.
"So we've got to work with the tools we have - the tools we have are diplomatic.
"I think the most powerful weapon we have at the moment is our ability to make president Putin and the Russians feel the consequences of what they are doing."
Mr Johnson also said Vladimir Putin's decision to pull out of a planned meeting in Paris after the French suggested the meeting should become a discussion about Syria, was "very significant".
"They are starting to feel the pressure and it is vital that we keep that pressure up," Mr Johnson said.
"There are a lot of measures that we are proposing to do with extra sanctions on the Syrian regime and their supporters, measures to bring those responsible for war crimes before the International Criminal Court.
"These things will eventually come to bite the perpetrators of these crimes and they should think about it now."
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