United Nations investigators have accused the Syrian regime of crimes against humanity, indicating President Bashar al-Assad himself should face prosecution.

A commission of inquiry answering to the UN Human Rights Council said it had compiled a confidential list of those "up to the highest levels" who had ordered the shooting dead of unarmed women and children, shelling of residential areas and torturing of wounded protesters in hospital.

It released its findings on Thursday as Western and Arab League diplomats and Syrian opposition figures gathered in Tunis to hammer out proposals to put pressure on President Bashar al-Assad to quit. Kofi Annan, the former UN chief and Nobel Peace Prize winner, was appointed joint envoy for the talks.

One option reportedly under consideration by the "Friends of Syria" was presenting a 72-hour ultimatum to Mr Assad that diplomats said would include as yet unspecified punitive measures, likely to include toughened sanctions.

According to a leaked draft declaration, the meeting will call on Syria to implement an immediate ceasefire to allow aid groups to deliver relief supplies to areas worst hit by the violence.

It also "recognised the Syrian National Council as a legitimate representative of Syrians seeking peaceful democratic change", a phrase which appeared to fall short of full endorsement of the most prominent group opposed to Mr Assad.

Tunisia´s presidential spokesman meanwhile said the host country would propose a political solution to the crisis involving a peacekeeping force.

Adnan Mancer, a presidential spokesman, said Tunisia would propose to the "Friends of Syria" conference for a Yemen-style transition, where the president stepped down.

He said Tunisia was ready to take part in the peacekeeping force to back "a political solution because we totally oppose a foreign military intervention."

Urgent discussions on precisely what challenge to present to Damascus continued throughout the day on the sidelines of the London Conference on Somalia between William Hague, the Foreign Secretary, Hillary Clinton, the US Secretary of State, and officials from about a dozen of the more than 70 nations and international organizations expected at the Tunis meeting.

Although more than 7,000 people are believed to have been killed since the uprising against Mr Assad 11 months ago, Britain and the US continue to insist that military intervention is not on the agenda, despite the pleas of Syrian opposition exiles.

Mr Hague said he would be arguing for more specific measures to be taken, without providing details, adding to speculation that the provison of arms or logistical assistance to the rebels was beginning to be discussed.

"I think part of that has to be tightening a diplomatic and economic stranglehold on the Assad regime," Mr Hague told the BBC.

At least 52 people were killed yesterday in Syria, the youngest being a four-year-old girl. Tanks were seen advancing into Baba Amr, the district where Marie Colvin, the Sunday Times journalist, and Remi Ochlik, a French photographer, were killed on Wednesday, signalling the possible beginning of a ground offensive to reclaim the district from the lightly-armed rebels holding it.

The international outrage caused by the mounting death toll has so far failed to persuade Mr Assad to call a halt to the offensive which appears aimed to crush the opposition into submission.

He cannot be referred to the International Criminal Court so long as he has the backing of Russia and China, who can use their veto in the UN Security Council.

But the UN inquiry´s findings, handed to the UN Human Rights Commissioner, Navi Pillay, will put more pressure on those countries to come up with their own solution to the crisis.

It said four intelligence and security agencies which report to Mr Assad personally "were at the heart of almost all operations" in putting down the uprising.

"A reliable body of evidence exists ... to believe that particular individuals, including commanding officers and officials at the highest levels of government, bear responsibility for crimes against humanity and other gross human rights violations," it concluded.

Western ministers will be eager to hear the proposals of the Arab League on how to increase pressure on Syria. The League has threatened sanctions against the regime, and set out its own proposals for a transition of power which Damascus has rejected.

As with Libya, the US, Britain and France, the three leading voices, will be reluctant to go further on recognising the opposition or supplying weapons without Arab League support.

The most likely scenario, should that come about, is for the Nato powers to provide logistical and humanitarian assistance to the rebels, while Arab League powers such as Saudi Arabia and Qatar supply arms.

A US State Department official was asked on the question of arms supplies: "Are you guys going to talk about it with the door closed and just not tell anybody?"

He replied: "What has brought together so many countries and organizations is sending a unified message to Bashar al-Assad that he´s on the wrong path, is this desire for a political solution and to respond to the immediate [needs] of the Syrian people." (© Copyright of Telegraph Media Group Limited 2012. 02/24/12)