David Cameron warns Syria of 'day of reckoning'
Prime Minister David Cameron has accused Syria's "criminal regime" of "butchering its own people".
He warned supporters of President Bashar Assad to turn their backs on their leader or face "a day of reckoning" for their involvement.
Speaking in Brussels, he also urged Russia and China to look hard at the suffering in Syria and reconsider their support for the Syrian government.
Both countries vetoed an Arab-backed peace plan last month.
At a media conference held after an EU summit, Mr Cameron told reporters: "The Assad regime is butchering its own people.
"The history of Homs is being written in the blood of its citizens."
'Blood on hands'
His comments come as a Red Cross convoy of food and medical aid is about to be delivered to the Baba Amr area of the Syrian city of Homs after a month-long siege.
Mr Cameron said immediate humanitarian access - as demanded by the UN - was "the very least that must happen to bring immediate relief to those who are wounded or dying".
He said crimes committed by Assad loyalists would be documented, adding: "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."
Mr Cameron said there were differences between Libya - where the UN intervened - and Syria.
"We don't have in Syria a UN resolution that gives all necessary measures as we had in the Libyan case.
"But instead of focusing on what we are not yet able to do, I think we should focus on those things we can do in terms of building the strongest possible political and diplomatic alliance to pressurise this regime."
Earlier, Labour peer and former Foreign Office minister Lord Malloch-Brown called on the West to "take stock of what's happened".
"This is not Libya, there was not a rebel movement with the capacity to overthrow the regime.
"There is not a willingness with the West to support them, to tip things in their favour, as happened in Libya.
"So we are forced to negotiate an end to this. And I think the key message that everybody in Syria, supporters of the government and opponents needs to understand, is there will be accountability for this, as David Cameron said."
Russian Prime Minister Vladimir Putin has insisted Russia has "no special relationship" with the Syrian government and criticised the West for backing the Syrian opposition.
"Instead of encouraging parties to the conflict, it's necessary to force them to sit down for talks and begin political procedures and political reforms that would be acceptable for all participants in the conflict,'' he said.
Later on Friday, the International Committee of the Red Cross (ICRC) is expected to enter Baba Amr, which has suffered heavy bombardment by government forces in recent weeks.
On Thursday, opposition fighters - the rebel Free Syrian Army (FSA) - announced they were to "tactically" withdraw from the district. The Red Cross now plans to evacuate the wounded.
The FSA said 4,000 civilians had refused to leave their homes and it was withdrawing to save them from an all-out assault.
Of the 100,000 people who normally live in Baba Amr, only a few thousand remain. Many are without power and running low on basic supplies.