The rebel Free Syrian Army (FSA) has given the government of President Bashar al-Assad a 48-hour deadline to observe a UN ceasefire plan.
The FSA's Colonel Qassim Saadeddine said if there was no response by Friday lunchtime the FSA would consider itself "no longer bound by the.. peace plan".
The plan calls on government forces to withdraw to barracks.
On Wednesday, UN observers confirmed the discovery of 13 shooting victims near the city of Deir el-Zour.
Col Saadeddine said in a video published online that the government must "implement an immediate ceasefire, 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," he went on.
All of these are provisions of the peace plan put forward by UN-Arab League special envoy Kofi Annan, the BBC's Jim Muir reports from neighbouring Lebanon.
The FSA is poorly armed and no match for the heavy weapons and tanks of the regime forces, our correspondent says.
But the UN has confirmed that it continues to control significant parts of many towns and cities, as well as much of the countryside in some areas, he adds.
Speaking after a closed meeting of the UN Security Council in New York, the US ambassador to the UN, Susan Rice, said a further escalation of the conflict in Syria is the likeliest scenario now, with it also spreading to other countries in the region.
"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," Ms Rice said.
Her British counterpart, Mark Lyall-Grant, said that a third element, including jihadists, was now active in Syria. He called them "spoilers".
Hopes of a new initiative over Syria have at the council have been dented by Russia and China.
Russian Deputy Foreign Minister Gennady Gatilov said Moscow was "categorically against any outside interference in the Syrian conflict" because it would "only exacerbate the situation for both Syria and the region as a whole".
A spokesman for China's foreign ministry reiterated Beijing's opposition to military intervention in Syria or forced "regime change".
Russia's foreign ministry added in a statement that the co-ordinated series of expulsions of Syrian diplomats from Western countries in response to the killing of 108 people in Houla last week was "counterproductive".
Turkey is also expelling all Syrian diplomatic staff from Ankara, but consular staff in Istanbul will remain.
In another development, Syria's honorary consul in California, Hazem Chehabi, resigned, saying he could not continue in his post after the "barbaric" massacre in Houla.
"The president has to be responsible for the actions of his own government. Either you're committing those atrocities, and therefore you're guilty, or you're not preventing them from happening," Mr Chehabi told the BBC.
'Appalling and inexcusable'
The UN confirmed on Wednesday the discovery of 13 men's bodies near the eastern city of Deir al-Zour, with their hands tied behind their backs, some apparently shot in the head.
UN mission chief Maj Gen Robert Mood said he was "deeply disturbed" by the "appalling and inexcusable act".
Amid the reports of continuing violence, it appears Mr Annan's talks with President Bashar al-Assad and opposition figures in Damascus on Tuesday have had no immediate impact on the ground.
The UN Human Rights Council will hold a special session on Syria on Friday looking into the killings, officials said.
One of the organisations monitoring the violence, the UK-based Syrian Observatory for Human Rights, said 65 "martyrs" had died in incidents across Syria on Wednesday.
As many as 15,000 people have been killed since the revolt against the rule of President Bashar al-Assad began in March of last year.