U.S. Secretary of State Hillary Rodham Clinton on Thursday condemned the Syrian government for new reports of "simply unconscionable" violence, accusing President Bashar Assad of intensifying a crackdown of a national uprising that has already killed thousands.

Providing the first U.S. reaction to what Syrian opposition groups were describing Wednesday as a new massacre, Clinton said, "We're disgusted by what we see happening."

"The regime-sponsored violence that we witnessed again in Hama yesterday is simply unconscionable," she said. "Assad has doubled down on his brutality and duplicity, and Syria will not, cannot be peaceful, stable or certainly democratic until Assad goes."

One organization claimed pro-government militiamen killed at least 78, including women and children, in the central Hama province, with some stabbed to death and others burned. The exact death toll and circumstances remain impossible to confirm. Syria rejected the claims as "absolutely baseless," blaming armed terrorists for what it described as a smaller attack.

Some 13,000 people have been killed in 15 months of violence, according to opposition groups.

Speaking in Turkey after meeting foreign ministers and senior envoys from 16 European, Turkish and Arab partners, Clinton outlined a set of principles that she said must guide the world in trying to solve the crisis. Foremost among them, it seemed, was Assad's eventual ouster _ and departure from Syria.

The demand appeared unnegotiable as Washington and its allies seek to broaden diplomatic efforts and win Russian and Chinese support for a structured end to the four-decade Assad regime.

But with neither Moscow nor Beijing present at the closed-door meeting in Turkey late Wednesday, and both remaining hostile to the idea of global sanctions against the Syrian government or any Libya-style military intervention, prospects for a breakthrough were unclear.

Later Thursday, U.N. mediator Kofi Annan will propose tasking a group of world powers and key regional players, including Iran, to come up with a strategy to end the 15-month conflict, U.N. diplomats said. Meetings could also take place on the sidelines of this month's meeting in Mexico of leading rich and developing nations, including a possible tete-a-tete between President Barack Obama and Russian President Vladimir Putin.

Clinton acknowledged that the U.S. and its partners haven't been successful yet in bringing through the kind of international action to end the violence. But she urged nations to remain united.

"We have to do more to help organize and focus the opposition," she said and called on the anti-Assad forces to do more themselves to coalesce around the idea of a democratic, representative and inclusive post-Assad future.

She said she was sending a senior envoy, Fred Hof, to Moscow on Thursday for talks with the Russian government, and she'll meet with Annan on Friday. Russia and China have twice blocked U.N. sanctions against Syria and vowed this week during a meeting between Putin and Chinese President Hu Jintao to oppose any regime change for the country.

Clinton also attended a counterterrrorism conference in Istanbul, pledging $15 million in assistance for transition countries seeking to improve their criminal justice sectors and rehabilitate and reintegrate violent extremists in prisons.

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Associated Press writer Christopher Torchia contributed to this report.

U.S. Secretary of State Hillary Rodham Clinton says Syria's recent violence is "simply unconscionable" and is blaming President Bashar Assad for "doubling down" on his harsh crackdown.

Referring to reports of a new massacre in central Hama province, she says Syria cannot be a peaceful, stable and democratic country until Assad leaves power.

Speaking in Turkey, Clinton says she and more than a dozen top foreign officials she has met have agreed on a set of principles for a new Syria.

They include the need for Assad to leave power and depart Syria _ explicitly calling for Assad's exile.

Syrian opposition groups say scores died in new violence on the outskirts of Hama. One organization claims pro-government militiamen killed at least 78. Syria rejects the claims as "absolutely baseless."