Col. Qassim Saadeddine of the Free Syrian Army said if officials in Damascus fail to end the violence by midday Friday, the opposition group would consider itself "no longer bound by the ... peace plan," the BBC reported.
The government, Saadeddine said in a video published online, must "implement an immediate cease-fire, withdraw its troops, tanks and artillery from Syrian cities and villages."
"It should also allow immediate humanitarian aid to all affected areas and free all detainees ... . The regime should also enter into a real and serious negotiation through the United Nations to hand over power to the Syrian people."
The British news network said Susan Rice, the U.S. ambassador to the United Nations, voiced fears the conflict could spiral further out of control.
"Members of this council and members of the international community are left with the option only of having to consider whether they're prepared to take actions outside of the Annan plan and the authority of this council," Rice said following a closed meeting of the U.N. Security Council in New York.
The bodies of 13 people who were bound and shot were discovered in eastern Syria, the head of the U.N. observer mission said Wednesday.
The discovery of the bodies came after Friday's massacre of more than 100 people, many of them children, in Houla, leading to worldwide outrage and condemnation.
A statement from the office of Maj. Gen. Robert Mood, chief military observer and head of the U.N. Supervision Mission in Syria, said the 13 bodies were found Tuesday night with their hands tied behind their backs in the Assukar area in eastern Syria. Some appeared to have been shot in the head from close range, the statement said.
Syria's honorary consul in California, Hazem Chehabi, quit his post over what he called the "barbaric" massacre in Houla.
"The president has to be responsible for the actions of his own government," Chehabi said. "Either you're committing those atrocities, and therefore you're guilty, or you're not preventing them from happening."
Government forces battled Syrian rebels Wednesday in Damascus, a witness said.
Violence was also reported in the Damascus countryside, Aleppo and Hama. Opposition leaders said at least 10 people were killed, including five in Douma, CNN reported.
Shelling destroyed homes in Homs, said the Syrian Observatory for Human Rights, an opposition group based in London, which said shelling was reported in the cities of Douma, Haish and Kafromah.
U.N.-Arab League envoy Kofi Annan left Syria Wednesday after meeting with Assad in Damascus Tuesday, Voice of America said. A U.N. official said Annan left without securing any major steps from the Syrian government to implement his faltering peace plan.
During his meetings, Annan renewed an appeal to the Assad regime to end the violence against its people.
"We are at a tipping point," Annan said. "The Syrian people do not want the future to be one of bloodshed and division. Yet the killings continue and the abuses are still with us today."
The United States, Japan, the Netherlands, Australia, Britain, France, Germany, Italy, Spain, Bulgaria and Canada said they are expelling Syrian envoys.
In another development, Russia and China repeated their opposition to military intervention in Syria, a day after French President Francois Hollande said he wouldn't dismiss such action if the U.N. Security Council approves it, VOA reported.
Assad has maintained the attacks were the work of "terrorist groups," Syrian state-run TV reported.
However, residents in Houla said Syrian regime forces terrorized the suburb of Homs, an anti-government hotbed. A U.N. human rights official said the majority of victims appeared to have been executed.
The United Nations said more than 9,000 people have been killed and tens of thousands of people have been displaced since anti-government protests began in March 2011. Opposition groups report a death toll of more than 11,000 people.
The nations that expelled Syrian envoys plan to push for tougher sanctions, British officials said, as the U.N. Security Council was to meet on the Syrian crisis.
The European Union was separately considering "a further tightening of sanctions on Syria," British Foreign Secretary William Hague said.
Syrian teams signed up for the 2012 Summer Olympic Games in London in July and August, could be banned, Hague said Tuesday.
Syrian Olympic Committee Chairman Mowaffak Jomahas said Britain had no right to ban Syrian athletes, but Hague said it would "be up to us who comes into the United Kingdom."
Military intervention in Syria is not being considered, Hague said.
"There is no support, no unanimity, about any intervention in the U.N. Security Council," he said in remarks cited by the Los Angeles Times. "All our efforts are going to support the Annan plan and to try to bring about a peaceful transition."