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The nations that make up the Friends of Syria group will call Friday for the United Nations to start planning a civilian peacekeeping mission in Syria that would begin after the regime agrees to a cease-fire and a political transition, a senior diplomat said.

The United States, European and Arab nations are demanding that Syrian President Bashar Assad immediately halt all violence and allow humanitarian aid into areas hardest hit by his regime's brutal crackdown on opponents, or face a tightening noose of international isolation and sanctions and an increasingly emboldened and powerful armed resistance.

"If the Assad regime refuses to allow this life-saving aid to reach civilians, it will have even more blood on its hands," U.S. Secretary of State Hillary Rodham Clinton said in opening remarks to an international conference of the Friends of Syria group.

"So, too, will those nations that continue to protect and arm the regime. We call on those states that are supplying weapons to kill civilians to halt immediately," Clinton said. She was referring to China and Russia, which have blocked U.N. action on Syria twice previously.

Assuming Assad agrees now, after ignoring numerous previous similar demands, the U.N. and Arab League would send in a joint peacekeeping force made up of civilian police officers with the permission of the ruling authority in Syria, whether it is Assad or a successor.

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The Friends of Syria, meeting for the first time, have no more leverage than in previous efforts, either as individual nations or through the United Nations, to make Assad leave. But the diplomat, speaking on condition of anonymity because sensitive diplomacy is ongoing, said the demand by the nearly 70 nations involved in the group will simply increase pressure on Assad and his allies to see that his demise is inevitable.

"The Assad regime has ignored every warning, squandered every opportunity, and broken every agreement," Clinton said.

The conference opened with senior Mideast officials condemning the Syrian government and demanding implementation of an Arab League plan that would see Assad step down in a peaceful transition to democracy.

Tunisian President Moncef Marzouki, the host, called for an Arab peacekeeping force to ensure stability during the transition.

"We have to respond to the demand of the majority of the Syrian people to get rid of a corrupt, persecuting regime," he said. "We have to stop the bloodshed, but this cannot be through military intervention."

Language in a draft final statement from the conference will allow U.N. Secretary General Ban Ki-Moon to begin recruiting nations to join the peacekeeping force and start identifying its mandate.

The plan is also designed to signal Russia and China that their continued support of Assad could leave them out of business and diplomatic opportunities in what the group hopes will be a new Syria.

Qatari Foreign Minister Jassim bin Jabr al-Thani, who has been a driving force to unite Arab opinion against the Syrian regime, directly called on Assad to step down.

Together with the Turkish Foreign Minister Ahmet Davutoglu, al-Thani called for the creation of humanitarian corridors to get aid to embattled citizens. They also called for a recognition of the Syrian National Council as an opposition force.

"We should concentrate our efforts on empowering the Syrian opposition, which will be the backbone of a new democratic regime," Davutoglu said, echoing calls from the other participants to bolster the alternative to Assad's regime.

The draft conference statement refers to the Syrian National Council as "a legitimate representative" of the Syrian people but leaves open the possibility that other opposition groups could emerge.

British Foreign Secretary William Hague described the Syrian government as a "criminal regime" as he arrived at the conference. He, Clinton and others were meeting with council members in a boost to their stature.

"We will ... intensify our links with the opposition," Hague said. "We will treat them and recognize them as a legitimate representative of the Syrian people."

Alexei Pushkov, a Russian lawmaker, said after meeting Assad that the Syrian president sounded confident and demonstrated no sign he would he step aside. Pushkov warned that arming the Syrian opposition would fuel civil war.

For their part, the Syrian National Council has welcomed the conference as part of their call for a peaceful transition to a democratic regime.

"This conference will help the Syrian people, the revolutionaries, I think; they will give us the power as a national council, a political umbrella for the revolution inside Syria," said Haithem al Maleh, executive director of the group.

As the conference began Friday, about 200 pro-Syrian demonstrators tried to storm the hotel. The protest forced Clinton to be diverted briefly to her hotel.

The protesters, waving Syrian and Tunisian flags, tussled with police and carried signs criticizing Clinton and President Barack Obama. They were driven out of the parking lot by police after about 15 minutes.

On Thursday, the Friends of Syria worked out details of the demands in London as the former UN chief, Kofi Annan, was named to be a joint U.N.-Arab League envoy to deal with the crisis.
Russia and China reiterated their opposition to an international resolution. Both nations say they support a "speedy end" to the violence, but they have vetoed two U.N. Security Council resolutions backing Arab League plans aimed at ending the conflict and condemning Assad's crackdown.

Diplomats said Assad's continued failure to comply with international demands would result in tougher sanctions and predicted that his opponents would grow stronger unless he accedes and accepts a political transition that would see him leave power.

Clinton and others ruled out any overt, direct lethal military aid to Assad's opponents, but her comments indicated that such steps were at least being considered if not already being done.

A draft of the Tunis conference's final 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, Syria's third-largest city, has been under a fierce government attack for nearly three weeks.