Boris Johnson has accused Russia of a "cynical and barbaric protraction" of the violence in Syria.
As the besieged city of Aleppo suffered its heaviest air strikes in months, the Foreign Secretary said Moscow was waging a "proxy war" in the country.
Speaking in New York where he has been attending the United Nations, he urged the Russians to end the conflict by forcing their ally, Syrian president Bashar Assad, to the negotiating table.
"What is absolutely clear is that the Russians bear the moral responsibility for what is taking place," he told Channel 4 News.
"You have got aid convoys with UN markings on them being targeted, you have got medical facilities being blown up.
"This is not a civil war, this is a proxy war conducted by puppeteers and those puppeteers are principally in the Kremlin.
"They have an opportunity to bring an end to this. They could tell the Assad regime not to fly, not to bomb, to engage with the whole negotiation.
"Instead, I'm afraid you're seeing a cynical and barbaric protraction of violence and it is tragic."
His comments came as US secretary of state John Kerry and Russian foreign minister Sergey Lavrov were preparing to meet in an attempt to revive the shattered ceasefire.
On Wednesday Mr Kerry bitterly denounced the Russians in the UN Security Council over the bombing of a UN aid convoy.