U.S. Secretary of State Hillary Clinton on Thursday said  the United States was willing to work with all members of the UN Security Council, which includes Russia, on a conference on Syria's political future.

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Speaking in Istanbul, Clinton urged Syria's President Bashar Assad to hand over power and leave his country, condemning a massacre near the town of Hama that was blamed on his supporters as  "simply unconscionable."

Regarding a possible conference over Syria's future, Clinton said it would have to start with the premise that Assad and his government give way to a democratic government, she told a news conference.

"We are disgusted by what we are seeing," she said, referring to continuing violence in Syria.

Ferce fighting broke out in the outskirts of Syria's capital, Damascus, and surrounding suburbs late on Wednesday, anti-government activists said, only hours after troops loyal to Assad had been accused by opponents of a new massacre of at least 78 people.

As violence continued, more than 55 mostly Western and Arab countries opposed to Assad's rule said that at a minimum an asset freeze on senior Syrian government officials as well as restricted business with Syria's central bank and leading commercial bank was needed to isolate his regime from the global financial system.

The group also expressed support for taking steps toward a United Nations Security Council "Chapter 7" resolution - a measure that could authorize the use of force.

"A continued concerted multinational approach is necessary to further deprive the Assad regime of the financial resources needed to sustain its campaign of violent repression," the Friends of Syria working group said at the end of a one-day meeting in Washington that was hosted by the U.S. Treasury.

Meanwhile, on Thursday, International mediator Kofi Annan was set to present the UN Security Council with a new proposal in a last-ditch effort to rescue his failing peace plan for Syria.

Last week, the New York Times reported that U.S. President Barack Obama is working to convince Russia to join an effort to implement a Yemen-style transfer of power in war-torn Syria.

According to Obama's reported plan, Syrian President Bashar Assad would step down from power, while several members of his regime would remain in office, as was the case with ousted Yemen President Ali Abdullah Saleh earlier this year.