Boris Johnson has summoned the Russian and Iranian ambassadors to the Foreign Office to tell them they deserve no credit for the evacuation of civilians from shattered eastern Aleppo in Syria.
The Foreign Secretary told the diplomats of the UK's "deep concern" at reports of civilians being executed, ambulances being shot at and "disappearances" carried out by pro-Bashar Assad forces.
He told them Russia and Iran were prolonging the suffering of Syrian people through their support for the Assad regime.
The meeting came as the first evacuation of civilians began from eastern Aleppo.
It follows a ceasefire deal which has seen the rebels surrender their last areas of control in Aleppo to the regime following a devastating ground and air offensive backed by Moscow and Tehran.
Following the meetings with the ambassadors, Mr Johnson said: "I summoned the Russian and Iranian ambassadors to the Foreign Office in order to convey in person the Government's profound concern over the situation in Aleppo.
"Both Russia and Iran have failed to uphold their obligations under international humanitarian law, specifically by failing to facilitate the delivery of humanitarian aid to civilians during the months when eastern Aleppo was besieged.
"They deserve no credit for the fact that an evacuation appears to be under way today.
"Having inflicted such suffering on the people of eastern Aleppo, Iran and Russia cannot expect praise for allowing some people to escape at the final hour.
"Both countries need to ensure the UN now oversees the evacuation process and that all civilians and non-combatants are properly protected."
The UN has warned that women and children are among those who have been targeted in planned massacres as pro-regime forces retake control of Aleppo.
It comes as Theresa May told a Brussels summit that the European Union must "robustly condemn" the actions of Assad, Russia and Iran.
The Prime Minister believes the EU faces a test of its credibility in its response to the crisis, a Number 10 source said.
Meanwhile, Defence Secretary Sir Michael Fallon condemned the brutality of pro-regime forces in Aleppo, and restated the UK's position that it sees no future for Assad in a post-war Syria.
Speaking at a summit in London of the international coalition fighting the Islamic State terror group, Sir Michael said: "We don't see a future for president Assad in Syria, even if he defeats the opposition in Aleppo. There is no victory in bombing hospitals, in restricting humanitarian aid and ending up in a country that you only control 40% of and is half destroyed, with millions dispersed and hundreds of thousands killed.
"We continue to work for a political settlement in Syria that is genuinely pluralist and can involve all sectors of Syrian society, but not Assad himself."
Assad has said "history is being made" with the defeat of rebels in Aleppo - a crucial battleground in the five-year conflict.
In a video message posted on the Syrian presidency's Telegram, Assad said that "what is happening is bigger than congratulations".
Elsewhere, Jeremy Corbyn insisted he will not stop buying a left-wing newspaper after it sparked outrage with a headline saying Aleppo was being "liberated".
Mr Corbyn said he disagreed with the front page headline in The Morning Star which stated Final Liberation Of Aleppo Is In Sight, but would not cancel his subscription to the publication.
"I did not agree with it at all. My view is, and always has been, and always will be, there has to be a political settlement in Syria," he said while visiting a Centrepoint homeless hostel in south London.
Asked if he would now cancel his subscription to the paper, Mr Corbyn said: "Listen, I buy lots of newspapers. I've written for The Morning Star, I read The Morning Star, and I have written for, and read, other newspapers.
"I frequently disagree profoundly with headlines, even in The Guardian, the Telegraph, the Mail, and so on, does it mean I won't buy them, or read them? Of course not.
"I disagree with that statement. I have written for, and no doubt will probably write again, for The Morning Star at various points in the future.
"They did, to their credit, give a very full report of a speech I made on human rights last Saturday."