The U.S., Europe and Arab nations Thursday crafted a stern warning to Syrian President Bashar Assad: Agree to an immediate cease-fire and allow humanitarian aid into areas hardest hit by his regime's brutal crackdown on opponents, or face as-yet unspecified punishments.

U.S., European and Arab officials met in London to work out details of an ultimatum to Assad. Diplomats said it would demand immediate compliance or result in additional punitive measures, likely to include toughened sanctions. The ultimatum is to be presented at a major international conference on Syria set for Friday in Tunisia.

A draft of the document obtained by The Associated Press calls on "the Syrian government to implement an immediate cease-fire and to allow free and unimpeded access by the United Nations and humanitarian agencies to carry out a full assessment of needs in Homs and other areas."

Homs has been under a fierce government attack for nearly three weeks.

The draft, which is still subject to change, also demands "that humanitarian agencies be permitted to deliver vital relief goods and services to civilians affected by the violence." More than 5,400 people have been killed in the nearly year-old uprising.

Meanwhile, Tunisia's presidential spokesman, Adnan Mancer, told The Associated Press in an interview ahead of Friday's meeting that the North African country will propose a political solution to the Syrian crisis that includes the deployment of a peacekeeping force and Assad stepping down from power.

The political transition would be akin to what happened in Yemen, where president Ali Abdullah Saleh stepped down in favour of his deputy after widespread protests. The Arab League already has made similar calls on Assad.

American officials accompanying U.S. Secretary of State Hillary Rodham Clinton to the Tunis meeting of the so-called Friends of Syria said the goal is to make it clear to Assad that his regime has a moral obligation to end the shelling of civilian areas and allow assistance into the country. The message will be that the burden is on Assad to respond to the demands of the international community, they said.

The officials spoke on condition of anonymity due to the sensitivity of the ongoing discussions over how the ultimatum will be presented at the Tunis conference. Several nations have proposed creating protected corridors through which humanitarian relief can flow but it was not clear if a consensus could be reached on the matter, as such a step would almost certainly require a military component.

More workable, officials said, would be a ceasefire such as the one proposed by the International Committee of the Red Cross, which is calling for a daily two-hour break in fighting to provide aid.

"Our priority is to facilitate the delivery of the humanitarian assistance, and we have some proposals on the table to prepare the best way to deliver this assistance as quickly as possible — if the regime would accept to open the country to (outside groups) or to the Red Cross," French Foreign Minister Alain Juppe said after the discussions.

Goal is to isolate Assad regime: Clinton

Clinton met Thursday in London with foreign ministers and senior officials from about a dozen countries, including Britain, France, Germany, Jordan, Morocco, Qatar, Saudi Arabia, Turkey, Tunisia and the United Arab Emirates. More than 70 nations and international organizations are expected at the Tunis meeting.

'I think that the strategy followed by the Syrians and their allies is one that can't stand the test of legitimacy... for any length of time.'— U.S. Secretary of State Hillary Clinton

"The efforts that we are undertaking with the international community ... are intended to demonstrate the Assad regime's deepening isolation," Clinton told reporters. "Our immediate focus is on increasing the pressure. We have got to find ways of getting food, medicine and other humanitarian assistance into affected areas. This takes time and it takes a lot of diplomacy."

If Assad doesn't comply, "We think that the pressure will continue to build. ... I think that the strategy followed by the Syrians and their allies is one that can't stand the test of legitimacy ... for any length of time," Clinton said. "There will be increasingly capable opposition forces. They will from somewhere, somehow find the means to defend themselves as well as begin offensive measures."

White House spokesman Jay Carney, travelling to Florida with President Barack Obama on Air Force One, told reporters the administration still opposes military intervention but "obviously we'll have to evaluate this as time goes on."

Other nations agreed that a military solution would be a last resort.

"There is no military option at the moment on the table, and as I have said before, France could not envisage such an option without an international mandate. It's a clear and constant guideline," Juppe said.

Russia, China confirm position against foreign interference

Russia and China, the two nations that blocked an earlier attempt at a UN Security Council resolution against Assad, reiterated their continued opposition Thursday to any foreign intervention in Syria.

Russia and China have vetoed two Security Council resolutions backing Arab League plans aimed at ending the conflict in Syria and condemning Assad's crackdown.

Russian Foreign Minister Sergey Lavrov's office said he called his Chinese counterpart on Thursday and they "reaffirmed the joint position of Russia and China."

Both countries support "a speedy end to any violence in Syria and the launch of inclusive dialogue between the authorities and the opposition without preconditions for a peaceful settlement that excludes foreign interference in Syrian affairs," the ministry said.

Tunisian spokesman Mancer also said his country was ready to take part in the peacekeeping force to back "a political solution because we totally oppose a foreign military intervention."

The Tunis conference will also explore ways to further isolate Assad and his inner circle as well as boost engagement with the Syrian opposition to help them prepare for an eventual democratic transition.

Warnings from Syrian activists of a humanitarian catastrophe in Homs grew more desperate Thursday as government forces resumed shelling an opposition stronghold in the city, where hundreds have died in a weeks-long siege.

As well, independent UN investigators called for called for Syrian officials to face prosecution for crimes against humanity, saying the UN had drawn up a confidential list of names of commanding officers and officials alleged to be responsible.

'Indiscriminate' machine-gun fire

Investigators have "credible and consistent evidence identifying high- and mid-ranking members of the armed forces  who ordered their subordinates to shoot at unarmed protesters, kill soldiers who refused to obey such orders, arrest persons  without cause, mistreat detained persons and attack civilian  neighbourhoods with indiscriminate tanks and machine-gun fire,"  says a report to the UN Human Rights Council.

Interviewed by CBC News on Thursday, the chairman of the UN's commission of inquiry into Syria said the list of senior Syrian commanders responsible for crimes against humanity remains highly confidential for now.

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Brazilian Paulo Pinheiro, who led an international investigation of allegations of human rights abuses in Syria, says it's not the commission's job to release names. (Anja Niedringhaus/AP Photo)

"We are not a tribunal," Brazlian Paulo Pinheiro said. "We are not a criminal court. We are not a criminal investigative body. What the human rights council requested us was to identify the responsibility of individuals, of state policies."

He said the naming of names would be a sort of indictment, and "this is not in our mandate." Instead, the material has been turned over to the office of the high commissioner.

"The Syrian people and all the international community will decide what institution is competent to deal with this information," Pinheiro said.

About 30 people, including two Western journalists, were killed in shelling on Wednesday — most of them in the rebel-held Baba Amr neighbourhood which is the centre of the resistance in the city. Homs has been under a fierce government attack for nearly three weeks now.

Homs-based activist Omar Shaker said intense barrages hit residential districts in Baba Amr again Thursday, but there was no immediate word on casualties. He said food, water and medical supplies are running dangerously low in Baba Amr.

"Every minute counts. People will soon start to collapse from lack of sleep and shortages in food," he said.

On Wednesday, shelling of Baba Amr killed American-born veteran war correspondent Marie Colvin and French photographer Remi Ochlik.