Prime Minister David Cameron said he hoped the "Friends of Syria" meeting would find ways to back the opposition and to tighten sanctions - and called on Russia and China to join the condemnation.

Syria will face demands to allow humanitarian access to the victims of its repression of anti-government protests, when more than 70 nations meet to discuss the crisis.

Foreign Secretary William Hague is among senior politicians gathering in Tunisia amid heightened global condemnation of the bombardment of Homs and other cities, 11 months into the violence.

They are expected to demand president Bashar Assad allow a ceasefire so that help can be got into the worst-affected areas as well as discussing ways to increase pressure on the regime.

Prime Minister David Cameron said he hoped the "Friends of Syria" meeting would find ways to back the opposition and to tighten sanctions - and called on Russia and China to join the condemnation.

Neither country is attending amid anger over their decision to veto a United Nations Security Council resolution backing an Arab League peace plan.

The UN last night appointed its former secretary-general Kofi Annan as a joint envoy with the Arab League on the Syrian crisis.

In a strongly-worded appeal for the world to unite to end the bloodshed, Mr Cameron seized on the death of Sunday Times journalist Marie Colvin as a signal of the regime's intent.

Her killing was not just a tragic loss but "yet another evil act by the Syrian government", Mr Cameron declared, accusing Mr Assad of murdering and maiming children in the crackdown.

"We do need to work hard to work out what more we can do as an international community," he told reporters yesterday, appealing directly to Moscow and Beijing.

"It's absolutely vital that the international community comes together, does this work, sends this message and I hope that the foreign ministers' meeting in Tunis tomorrow will back that up as strongly as they can."

Mr Hague said the UK was "determined to pursue every possible peaceful means of pressurising Assad's regime until it ceases its brutal repression of the Syrian people.

"The Friends of Syria group is an essential means of doing that. The meeting will show the breadth and strength of international support for the Arab League's efforts to resolve the crisis.

"The UK looks forward to working closely with Arab and international partners to end the violence and begin the process of transition that the Syrian people deserve."

US Secretary of State Hillary Clinton said she was certain the Syrian regime would fall but said getting aid in was a priority.

There was "great resolve and commitment" to find a way to end the violence, she said after attending a conference in London to discuss Somalia.

"Our immediate focus is on increasing the pressure. We have got to find ways of getting food, medicine and other humanitarian assistance into those affected by violence. But this takes time and it takes a lot of diplomacy," she told reporters.

"The strategy followed by the Syrians and their allies is one that can't stand the test of legitimacy or even brutality for any length of time.

"There will be increasingly capable opposition forces. They will from somewhere, somehow, find the means to defend themselves as well as begin offensive measures.

"It is clear to me there will be a breaking point. I wish it would be sooner, so that more lives would be saved, than later, but I have absolutely no doubt there will be such a breaking point."