At a meeting in Cairo, the first time the Arab League had hosted the Syrian opposition, Arab League Secretary-General Nabil el-Araby said there is "an opportunity before the conference of Syrian opposition today that must be seized, and I say and repeat that this opportunity must not be wasted under any circumstance.
"The sacrifices of the Syrian people are bigger than us and more valuable than any narrow differences or factional disputes."
The Syrian Network for Human Rights and Damascus Center for Human Rights Studies reported Monday 30 casualties in Syrian cities, most of them in a massacre in the town of Douma in Hama.
Meanwhile, Syria's opposition snubbed a transition plan that doesn't demand President Bashar Assad's ouster and U.S. Secretary of State Hillary Clinton said may fail.
The compromise agreement worked out by world and regional powers in Geneva, Switzerland, is "ambiguous" because it leaves open the possibility Assad will be part of an interim administration, opposition Syrian National Council spokeswoman Bassma Kodmani said.
The deal says the called-for transitional national unity government would be decided by "mutual consent," which would give the Assad regime, the opposition and other groups veto power, she said.
It also includes no means or schedule for implementation, she said.
"We cannot say that there is any positive outcome today," Kodmani said Sunday.
"If we don't have firmness and clarity, we have more victims," she said.
The agreement worked out Saturday included a text change on which Russia insisted, diplomats said.
The original agreement, prepared by U.N.-Arab League envoy Kofi Annan, called for a transitional administration that would exclude "those whose continued presence and participation would undermine the credibility of the transition, and jeopardize stability and reconciliation," which diplomats said meant Assad.
But Russia objected, leading to the phrase's replacement with the "mutual consent" stipulation.
The meeting brought together foreign ministers and comparable officials from the five U.N. Security Council members -- the United States, Britain, France, Russia and China -- plus regional powers Turkey, Iraq, Kuwait and Qatar. Arab League and European Union officials also participated.
Syrian state media called the Geneva meeting a victory for the Assad regime and longtime ally Russia.
"Today we are witnessing the birth of a new school in international diplomacy," commentator Abdel-Hadi Nasri said on Syrian state TV.
Russian Foreign Minister Sergei Lavrov said he was "delighted" by the Geneva outcome.
He repeated Moscow's position there should be no preconditions to Syria's transition process and no attempt to exclude any group from a proposed transitional government.
Clinton said the text change did not alter the agreement's substance and Assad would never pass the test of mutual consent.
But she said she was not confident the agreement would succeed.
"There is no guarantee that we are going to be successful. I just hate to say that," Clinton told CNN.
British Foreign Secretary William Hague told the BBC Sunday he believed Assad would be excluded from any unity government under the agreement's terms.
French Foreign Minister Laurent Fabius told French TV channel TF1, "The opposition will never agree to [Assad], so it signals implicitly that Assad must go and that he is finished."
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