Russia was directly involved in the bombing of a UN aid convoy on the outskirts of Aleppo in a "revenge" attack for an earlier air strike that killed 60 Syrian soldiers, according to senior coalition officials.
Eyewitnesses on the ground reported seeing a number of Russian surveillance aircraft circling overhead before the aid convoy was struck by a series of missiles fired by warplanes, killing an estimated 20 civilians and destroying 18 UN trucks carrying vital humanitarian supplies to besieged rebel areas.
Speaking at the UN General Assembly in New York, Theresa May said she "condemned" the attack, which has exacerbated fears that the week-old US-Russian sponsored ceasefire is on the point of collapse.
The Prime Minister also used her first major foreign policy address to announce billions of pounds of funding to the Middle East and Africa designed to stop migrants travelling to Britain.
Boris Johnson, the Foreign Secretary, said: "The attack on the aid convoy in Aleppo was appalling and a clear violation of the most basic of humanitarian principles."
In a further suggestion of Russian involvement in the attack, its defence ministry on Tuesday said that the convoy had been accompanied by a militants' pickup truck armed with a heavy mortar gun, Russian news agencies reported.
The ministry uploaded to YouTube what it claimed was drone footage of the convoy which showed "new details" about the incident.
Russia, a key ally of Syria's President Bashar al-Assad, denied any involvement in the attack, which took place after the 31-truck convoy had parked in a compound to deliver vital supplies to rebel-held areas outside Aleppo.
The Syrian military also denied involvement.
But senior coalition officials last night (Tuesday) said they had evidence of Russian involvement in the attack, claiming it was launched in response to a coalition air strike in eastern Syria at the weekend, in which an RAF Reaper drone took part, which killed 60 Syrian soldiers. "All the evidence suggests the Russians were involved in bombing the aid convoy as an act of revenge for the coalition air strike at the weekend which killed 60 Syria soldiers," said a senior coalition official. "If this can be proved, then the Russians could find themselves facing war crimes charges."
The missiles and warplanes used in the attack were the same as those used by the Russians to attack rebel positions.
Coalition officials also strongly reject Russian claims that their aircraft deliberately targeted Syrian troops in last weekend's bombing. "We gave the Russians details of the area we were targeting where we had identified a group of Isil fighters operating," an official said.
"But the Russians failed to tell us there were Syrian government forces in the area until it was too late."
Under the ceasefire terms negotiated by US Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov, only groups associated with Islamic State in Iraq and the Levant (Isil) can be targeted.
In unusually outspoken remarks made in his final address to the UN General Assembly in New York, the secretary-general, Ban Ki-moon, deliberately pointed the blame for the attack on the aid convoy at Moscow.
Calling the attack on the convoy "sickening, savage and apparently deliberate", he launched a stinging attack on the Syrian government, saying it had killed the most civilians in the civil war that has lasted more than five years.
And in a clear reference to Russia he said, "powerful nations ... feeding the war machine, also have blood on their hands." Speaking after talks with Mr Lavrov on the sidelines of the General Assembly yesterday, Mr Kerry insisted the ceasefire deal was still "not dead".
Meanwhile, Mrs May used her first foreign policy speech to the UN to announce that billions of pounds will be poured into the Middle East and Africa to stop the rise of Isil and the flood of migrants into Europe.
She has pledged to spend pounds 660 million on supporting refugees in Syria and neighbouring countries Jordan and Lebanon.
The extra money, designed to prevent refugees attempting the journey to Europe, takes the total spent this year by the UK to pounds 1.5 billion.
In addition, millions of pounds in aid money will be used to send British troops on a training mission to Somalia.
Around 100 UK soldiers will travel to the African country to train local forces in the fight against al-Shabaab amid fears that terrorist groups including Isil are gaining traction in the region.
A further pounds 20 million will be spent trying to ensure Somalian refugees in Kenya are able to return to their home country.
And pounds 80?million will be spent to help provide 30,000 jobs for Eritrean refugees in Ethiopia.
The pledges are an attempt to reduce the flow of migrants attempting to get to European countries such as the UK.
"It's about how best to serve Britain's interests," a government official said.
Mrs May said: "This affects all of us, and it is the responsibility of us all to take action.
"We cannot ignore this challenge, or allow it to continue unmanaged. We need to do better. Better for the countries people leave, for the countries they move through, for the countries they try to get to - and most of all, better for the migrants and refugees themselves."