Boris Johnson has insisted it is "too early" to make "conclusive judgments" about who was responsible for an attack on an aid convoy in Syria.
Mr Johnson has met his Russian counterpart Sergei Lavrov for the first time to discuss the attack, which left 20 civilians dead and which the US has said it holds Russia responsible for.
US officials have told the BBC that two Russian jets were in the sky above the convoy at the moment it was hit. But Moscow has rejected the accusation.
The Foreign Secretary said Russia and Syria were the only two potential "culprits" and the UK had "doubts" that Syria would be able to carry out such a strike at night.
Mr Johnson told the BBC that "this is not a civil war going on in Syria, this is a proxy war" as he was asked whether the suggestion that Russia may have committed a war crime made the prospect of reviving a ceasefire more difficult.
He said: "I think it's too early to say anything about criminality and that kind of thing, or indeed to make conclusive judgments about responsibility.
"But, put it this way, when you look at what happened to the aid convoy, there are only two possible culprits, only two forces that are capable of having carried out that strike flying in that area - they are the Syrians and the Russians, and we have our doubts about the Syrian capability to fly at night.
"You are left with a pretty strong conclusion as you'll have been hearing."
Mr Johnson met Mr Lavrov on Tuesday while the pair were in the US for the United Nations General Assembly.
Their face-to-face meeting in New York was their first since Mr Johnson was appointed, and came days after talks between Theresa May and Moscow over Russian president Vladimir Putin's support for Bashar Assad's Syrian regime.
Mr Johnson said the "overwhelming responsibility" for the failed ceasefire in the country rested with the Assad regime and its sponsors.
He said: "Everybody in that room knows that and it's up to people in that room to create the conditions for a ceasefire - they have done it before, they can do it again - and we know that the overwhelming responsibility for the failure to have a ceasefire, the overwhelming responsibility for the breaches in the ceasefire that we have seen lies with the Assad regime and indeed its sponsors."
But Mr Johnson said "we shouldn't give up" on the prospect of a resolution with negotiations in a "pretty critical state but it is not yet terminal and it can be revived".
While the US has blamed Russia for the attack on the aid convoy, Peter Ford, the former British ambassador to Syria, said there is "clearly much doubt" over who carried it out.
He told the BBC Radio 4 Today programme: "The United Nations, having initially spoken about air strikes have significantly rowed back, they now just talk about attacks of uncertain provenance.
"And the only evidence we have is what opposition spokespeople say."
He said the accidental bombing of Syrian regime forces by US troops just days ago shows that its intelligence can be wrong.
He said: "The United Nations has not come down on the side of saying they were air strikes, we just don't know."