Updated

LONDON -- The United States, Europe and Arab nations are preparing to demand that Syrian President Bashar Assad agree within days to a ceasefire and allow humanitarian aid into areas hardest hit by his regime's brutal crackdown on opponents.

U.S., European and Arab officials were meeting in London on Thursday to craft details of an ultimatum to Assad that diplomats said could demand compliance within 72 hours or result in additional as-yet-unspecified 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.

American officials accompanying U.S. Secretary of State Hillary Rodham Clinton to the Tunis meeting of the "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.
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,

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Tunisia and the United Arab Emirates. More than 70 nations and international organizations are expected at the Tunis meeting.

"It is a deeply frustrating situation," British Foreign Secretary William Hague told BBC radio ahead of the talks. He said that the Assad regime "has continued to act seemingly with impunity."

However, Hague said military intervention was very unlikely, as "the consequences of any outside intervention are much harder to foresee."

In addition to demanding that the government allow humanitarian access, the Tunis conference will 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.