Aleppo, Syria’s second city, is at risk of being “bombed into smithereens” by Russia unless a fresh ceasefire can be agreed with Vladimir Putin, the US secretary of state, John Kerry, has said.



But Kerry said he still believed a ceasefire could be agreed, and revealed proposals were still going back and forth between Moscow and the US.

Speaking in London on Monday, he said the diplomatic process was delicate and “very dangerous” and its success or failure would come down to Moscow’s true priorities.

“We will see whether Russia has a greater desire to bomb Aleppo into smithereens”, claiming they are hitting terrorists, rather than the accept the reality that “there is an opposition there prepared to live by the ceasefire,” he told the Chatham House thinktank.

“This will determine to some degree where we go in Syria in the longer term.”

A ceasefire could be agreed, he said, if Russia halted its bombing campaign long enough to allow countries backing the legitimate Syrian opposition to persuade those forces in Aleppo to separate themselves from Jabhat Fateh al-Sham, the al-Qaida-linked group formerly known as the Nusra front.

Displaced Syrians carry their belongings out of Aleppo on the third day of a rebel offensive. Photograph: Omar Haj Kadour/AFP/Getty Images

Kerry made his remarks as Russia sent naval reinforcements to the Mediterranean that western observers believe could be used for a fresh offensive on Aleppo.

As opposition forces attempted to break the siege of rebel-held eastern Aleppo with an assault heavily criticised by the United Nations, the Syrian government claimed that as many as 84 people had been killed and 280 wounded by opposition shelling over the past three days.

Russia claims it has not bombed the city for a fortnight, but military clashes have persisted throughout Aleppo. Moscow has said it will respond with all available means if necessary.

Kerry said he would work right up to the end of Barack Obama’s presidency to try to find a political solution to the Syrian problem, which he described as a contorted web.

He implied he did not believe that a perceived power vacuum in Washington in the weeks after the 8 November presidential election would give Russia a chance to capture eastern Aleppo, and so deliver a hammer blow to the Syrian opposition.

Kerry was unusually frank in admitting western diplomats had been unable to persuade the legitimate opposition not to fight alongside Jabhat Fateh al-Sham. If the militant group decided to attack the regime during a ceasefire, he said, “then what happens is the regular opposition gets swept up with them … and then all of a sudden your ceasefire starts to shred”.

Bashar al-Assad, he said, also played a major role in the breakdown of ceasefires by bombing legitimate opposition forces while claiming to go after Jabhat Fateh al-Sham and Islamic State (Isis). “So the opposition then get angry and say he is not showing good faith, because he is not, and it spirals downwards,” Kerry said.

Syria, he argued, was “a whole bunch of wars taking place at once in the same place”. It was “Kurd v Kurd, Kurd v Turkey, Iran and Saudi; you have got Turkey, Daesh [Isis], Persian Shia versus Arab Sunni, Sunni v Shia, a lot of people against Assad, you have the challenge of Shia Iraqi militia coming into Syria and then, of course, you have Hezbollah-supporting Assad. So this is is about as toxic a diplomatic cocktail I can think of.”