David Cameron has spoken of his frustration at Britain's inability to do more to to prevent the slaughter in Syria.
The Prime Minister said work was under way to ensure president Bashar Assad and his regime could be held legally responsible for the brutal repression.
But Britain had to be realistic about its capacity to intervene directly, he cautioned - insisting the situation was different from Libya.
Syria was sliding closer to all-out civil war today as rebels sought to more formally organise their resistance in the face of an intensified bombardment by government forces on the city of Homs.
The rapidly deteriorating security and humanitarian situation led the UK to announce that it had withdrawn its diplomatic staff from Syria and closed the Damascus embassy.
Foreign Secretary William Hague, announcing the decision to MPs, urged those fighting for the Assad regime to lay down their arms.
Mr Cameron, who Downing Street said would raise the issue at the European Council summit in Brussels, said the international community was trying to exert "maximum pressure on Assad and his dreadful regime".
"We've been pushing for resolutions at the United Nations we've been working with the Syrian opposition to try and encourage them be more inclusive," he told Channel 5 News.
"I think we have to keep up that pressure. But we do have to recognise ... there are big differences between the situation in Libya and the situation as Syria.
"The biggest difference, of course, being we had the express permission of the Arab League and the United Nations to take all possible measures, to take the military action that we took."
He hit out at Russia and China for blocking a United Nations Security Council resolution backing an Arab League peace plan as the pair also voted against a UN condemnation of human rights abuses.
"But above all we have got to hold Assad responsible," he said.
"There is a role for Britain and for others right now to start collecting the evidence to make sure we hold this man to account for what he does no matter how long it takes.
"Britain is doing this now, building a picture to show these are crimes against humanity and this man may need to be held to account for that.
"That is very important. It is extremely frustrating that we cannot do more. The scenes that we have seen on our televisions are appalling.
"I wish we could do more but we have to be realistic about what we can achieve about holding them to account. Gathering the evidence, using that evidence to build a case in international law that he can never hide from - that we can do."
Announcing the closure of the embassy, Mr Hague said staff and premises were judged at risk but that it "in no way reduces the UK's commitment to active diplomacy to maintain pressure on the Assad regime to end the violence".
The UK would continue to work with the opposition Syrian National Council and support UN and Arab League special envoy Kofi Annan's work, he said.
Remaining British nationals, who the Foreign Office has urged to leave, should contact the embassy of any remaining EU member state if they needed assistance, he said.
Britain also condemned the refusal by Damascus to allow the United Nations humanitarian affairs and emergency relief coordinator Baroness Valerie Amos into the country to assess the scale of suffering caused by the violence.
But Syrian authorities today did grant the International Committee of the Red Cross permission to enter the besieged Homs neighbourhood of Baba Amr.
Downing Street called on Syrian troops not to turn their weapons on civilians.
"There is a choice for those who have taken up arms against innocent civilians," said a Downing Street spokeswoman. "Give up your weapons and we will support you in building a freer Syria, or face the pressure of international opinion."
The British Red Cross has launched a Syria Crisis Appeal to help provide food, medical supplies and transport for the wounded.
The charity said it had provided more than £500,000 to support humanitarian work in Syria since May last year.