Geneva, Switzerland (CNN) -- There is no guarantee that a sweeping new international agreement on Syria will succeed in ending the conflict there, U.S. Secretary of State Hillary Clinton conceded, as opposition activists said the number of dead had skyrocketed in recent months.
"There is no guarantee that we are going to be successful. I just hate to say that," Clinton told CNN.
But she expressed optimism that a new agreement hammered out Saturday would help ease President Bashar al-Assad out of power.
The first plan backed by Russia and China as well as the West, it calls for a transitional government as a step towards ending the 16-month uprising.
Opposition activists immediately criticized the deal as leaving open the possibility that al-Assad would remain in power.
"The new agreement provides vague language which is open to interpretation," the opposition Local Coordination Committees of Syria said in a statement Sunday. "This provides yet another opportunity for the regime's thugs to play their favorite game in utilizing time in order to stop the popular Syrian Revolution and extinguish it with violence and massacres across Syria."
A spokesman for the Syrian National Council, a main political opposition group, similarly slammed the agreement.
"We are afraid that the decision of the Geneva convention might give signs and gestures to the Syrian regime that it is acceptable and a legitimate cover to continue killing the people, and committing more massacres," Muhammad Farmini told CNN.
"This gives the regime a permit to continue killing and spilling more Syrian blood," he said.
But Clinton said al-Assad and his inner circle would be excluded from any transitional government.
Both sides have to agree on the membership of the interim body, and Clinton said there was "no way anyone in the opposition would ever consent to Assad or his inside regime cronies with blood on their hands being on any transitional governing body."
"Assad will not be part of it," she said in an interview late Saturday in Geneva, Switzerland, after the deal was hammered out.
Kofi Annan, the joint special envoy for the United Nations and Arab League, invited diplomats from the U.N. Security Council and envoys from Turkey, the United Nations, the European Union and the Arab League to the global meeting in Geneva on Saturday.
Clinton said the Russians, who have long been al-Assad's most steadfast supporters, had finally decided to back a transition away from his rule.
"They have committed to trying," she said. "But they also admitted that they may or may not have enough leverage to convince not just one man, but a family and a regime that their time is over."
Bloodshed continued unabated in the wake of the international talks, with at least 69 people dying on Sunday, according to the opposition Local Coordination Committees of Syria.
It said Sunday that more than half of the people killed since the Syrian crisis began 16 months ago were slaughtered in the past four months.
More than 14,000 people have been killed by the regime, according to tallies from opposition activists.
June was a particularly gruesome month, with 2,386 people killed compared to 1,196 in May, the LCC said.
The dead on Sunday include several people who died from injuries sustained in a blast at a funeral procession in Zamalka on Saturday, the group said.
"Dr. Jamal Tabarneen was martyred by the regime's army gunfire while he was helping in resuscitating the injured in Zamalka's explosion," the LCC said.
The international group meeting in Switzerland agreed that both the regime and opposition fighters should immediately adopt a cease-fire and implement Annan's six-point peace plan without waiting for the actions of others, Annan said.
The group also called for a transitional government.
Annan said it could include members of the current Syrian regime, making it theoretically possible that al-Assad will be a part of the transition.
But Annan pointed out it is the Syrians who will decide the make-up.
"I think people who have blood on their hands are hopefully not the only people in Syria," Annan said. "I think the government will have to be formed through discussion, negotiations, and by mutual consent. And I will doubt that the Syrians -- who have fought so hard for their independence, to be able to say how they're governed and who governs them -- will select people with blood on their hands to lead them."
The LCC said it was "gravely concerned" by what could happen in a transitional period with al-Assad's military and security forces.
There are "core conflicts between the Revolutionaries and Bashar al-Assad's pillars and symbols, whose hands are stained with blood and must be excluded from the transitional phase," the opposition group said. "No Syrian will accept a regime waiver of responsibility over crimes and violations against human rights."
The agreement in Geneva also calls on the Syrian government to release detainees and allow journalists access to the country. The right to peaceful demonstrations must be respected, Annan said.
Clinton said the U.N. Security Council should endorse the plan, thus allowing the possibility of sanctions against Syria if the requirements aren't met.
Russia, widely viewed as a key ally to the Syrian regime, said the agreement should not be interpreted as outside powers imposing a transitional government on the Syrians. That process must come from inside Syria, Russian Foreign Minister Sergey Lavrov.
CNN's Saad Abedine and Holly Yan contributed to this report.