David Cameron accused Syrian president Bashar Assad’s “criminal” regime of “butchering its own people”.
The Prime Minister warned Assad’s supporters to turn their backs on the president or face a “day of reckoning” for their part in the “truly terrible” scenes in the country.
He also urged China and Russia to end their backing for the Syrian government.
“The Assad regime is butchering its own people.
“The history of Homs is being written in the blood of its citizens,” Mr Cameron said.
He described the situation in Homs, where rebels have faced a bloody crackdown from the regime’s forces, as “a scene of medieval barbarity”.
Immediate humanitarian access to the city – as demanded by the United Nations Security Council – was “the very least that must happen to bring immediate relief to those who are wounded or dying”.
“I say to the Russians and the Chinese: look hard at the suffering of Syria and think again about supporting this criminal regime,” he said.
Efforts to document the atrocities being perpetrated by the Assad loyalists were backed yesterday by the European Council in its communique on summit talks.
“We will make sure, as we did in Serbia, that there is a day of reckoning for those responsible,” Mr Cameron said.
“So I have a clear message for those in authority in Syria: make a choice, turn your back on this criminal regime or face justice for the blood that is on your hands.”
Russia and China, which vetoed an Arab-backed peace plan last month, are continuing to resist pressure to join global condemnation of human rights violations.
However, they have now backed a UN statement criticising the regime’s refusal to allow UN humanitarian chief Baroness Amos access to Syria to inspect the bloody aftermath of 11 months of violent repression.
More than 7,500 civilians have been killed so far in the crackdown, the UN estimates, and its top human rights body voted to condemn Syria for “widespread and systematic violations” though without Russian or Chinese support.
Rebel forces have made what they described as a “tactical retreat” from the besieged Baba Amr area of the city of Homs, which has become emblematic of the bitter struggle.
In a newspaper interview yesterday, Russian Prime Minister Vladimir Putin denied that Moscow had any special relationship with Damascus and said the Syrians must choose who governs them.
He said: “When Bashar al-Assad came to power he visited London and other European capitals first. We don’t have a special relationship with Syria.
“It is up to the Syrians to decide who should run their country. We need to make sure they stop killing each other.”
The withdrawal of rebels from Baba Amr came as the Foreign Office investigated the authenticity of videos posted by activists in Syria apparently showing Sunday Times reporter Marie Colvin and French photographer Remi Ochlik being buried.
A man in the videos says the pair, killed last week in a rocket attack on the besieged Baba Amr district of Homs, were interred in a local cemetery because power shortages meant their bodies could not be preserved.
A UK Government spokeswoman said: “We haven’t been able to get any confirmation that the video claiming to show Marie Colvin being buried is real.”
French president Nicolas Sarkozy said two French journalists, Edith Bouvier and William Daniels, have escaped to Lebanon after being trapped in Baba Amr .