The "barbaric cruelty" shown by Syrian regime forces, which have reportedly executed scores of civilians in the bombarded city of Aleppo, shows that president Bashar Assad has no place in the country's future, Downing Street has said.
Theresa May's official spokeswoman described as "extremely concerning" reports that 82 civilians have been killed "on the spot" as Russian-backed Syrian troops retake the last of previously rebel-held areas in east Aleppo.
The United Nations relaying of the reports was accompanied other dispatches of mass killings, reinforcing fears that regime forces are committing atrocities as they approach victory in the crucial battleground.
The Syrian military said it now holds 99% of the former rebel territory and denied reports of summary killings.
But in an emergency debate in the House of Commons, the events have been compared to genocides in Srebrenica and Rwanda.
Leading the session, former cabinet minister Andrew Mitchell said Britain is "complicit" in the suffering faced by thousands of civilians in besieged Aleppo.
He read out the words of a resident of the city, who called on the UK to help establish a humanitarian corridor to evacuate civilians and said "nobody will ever believe" Britain again over fighting terror if it ignores "state terror".
Former international development secretary Mr Mitchell added: "This country, along with the entire international community, 10 years ago embraced the responsibility to protect, a doctrine which said that nation states will not allow the Srebrenicas, the Rwandas and other appalling events, including in Darfur, to take place again.
"This responsibility to protect was signed up to at great fanfare and embraced by all the international community, great and small.
"Yet here we are today witnessing, complicit, in what is happening to tens of thousands of Syrians in Aleppo."
Mrs May's spokeswoman said Britain was working with EU countries to secure a "strong, clear statement" about "the need for humanitarian access and for a ceasefire" at the conclusion of Thursday's European Council summit in Brussels.
At a Westminster briefing, she went on: "There are some extremely concerning reports coming out from Aleppo of what is happening there.
"We have consistently throughout said that people should need to face consequences for their actions, that's some of the work that we have been doing with partners.
"We need the international community to come together and alleviate the suffering in Aleppo, and as we have consistently said, and if you look at some of the reports coming out of Aleppo, we do not think that president Assad, who is presiding over such barbaric cruelty to the people of Syria, is a route to a long-term secure, prosperous future for Syria.
"That's why we think there needs to be a political transition away from Assad."
As the UN's human rights office said it received reports of pro-government forces killing at least 82 civilians in four neighbourhoods of the rapidly-shrinking rebel enclave, including 11 women and 13 children, charities called on the UK to take action.
Save the Children said the situation was "catastrophic" with people's "worst fears" of revenge attacks becoming a reality.
Hundreds of children are believed to be "in the middle of the battlefield", the charity's Kirsty McNeill said.
She went on: "The warring parties are ultimately responsible for civilian deaths and suffering. But the international community, in particular the five permanent members of the (UN) Security Council, also bear a grave responsibility for what is happening in Aleppo.
"They have utterly failed to protect the city's children during months of siege, but there is at least now an opportunity for Britain to make every effort to end this carnage and safely evacuate the remaining civilians.
"The emergency debate in the House of Commons today on Aleppo's tragedy must be more than just words - we need to see urgent action by the UK to put pressure on the Russian government to ensure that safe routes are opened for civilians today."
The International Committee of the Red Cross (ICRC) said tens of thousands of civilians trapped in the city "have literally nowhere safe to run" and urged fighters to observe "the basic rules of warfare - and of humanity".
Unicef said it is concerned over the unverified reports of "extra-judicial killings of civilians, including children".
Russia criticised US president Barack Obama's administration, which is also calling for a ceasefire.
"We are tired of hearing this whining from our American colleagues in the current administration that we need to immediately halt military action," foreign minister Sergey Lavrov told journalists during a visit to Serbia.