David Cameron vowed today that Syrians involved in "butchering" their own people would face a "day of reckoning".
Supporters of president Bashar Assad's "criminal regime" have blood on their hands, the Prime Minister said.
He described the situation in Homs, where rebels have faced a bloody crackdown from Assad's forces, as "a scene of medieval barbarity".
Speaking at a post-summit press conference in Brussels, Mr Cameron urged China and Russia to end their support for the Syrian government, calling on them to "look hard at the suffering".
Immediate humanitarian access to the city - as demanded last night by the United Nations Security Council - was "the very least that must happen to bring immediate relief to those who are wounded or dying".
Mr Cameron added: "The Assad regime is butchering its own people. The history of Homs is being written in the blood of its citizens."
He added: "We will make sure, as we did in Serbia, that there is a day of reckoning for those responsible.
"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 backed a UN statement criticising the regime's refusal to allow UN humanitarian chief Baroness Amos access to Syria to inspect the aftermath of 11 months of violent repression.
"I say to the Russians and the Chinese: look hard at the suffering of Syria and think again about supporting this criminal regime," he added.
Russian prime minister Vladimir Putin denied that Moscow had any special relationship with Damascus and said the Syrians must choose who governs them.
"When Bashar al-Assad came to power he visited London and other European capitals first," he told The Times. "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."
Efforts to document the atrocities by Assad loyalists were backed by the European Council in its summit communique.
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 last night 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.
It 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, were interred in a local cemetery because power shortages meant their bodies could not be preserved.
"We haven't been able to get any confirmation that the video claiming to show Marie Colvin being buried is real," a UK Government spokeswoman said.
France has announced it is closing its embassy in Syria.