Negotiations between the Syrian regime and the opposition have broken down in Homs, the country's third largest city that has been widely devastated by government bombardments.
The opposition says government's demands to put down their arms in exchange for assuring civilians' safety are unacceptable and are calling for help from the international community.
The developments came as the UN envoy to Syria said any resolution to the fighting in the country must involve President Bashar al-Assad.
"President Assad is part of the solution," Staffan de Mistura said in Vienna, Austria, on Friday.
"I will continue to have very important discussions with him. The only solution is a political solution."
This was the first time a UN envoy on Syria has explicitly named Assad as part of a peaceful solution after nearly four years of fighting between government forces and rebels seeking his overthrow.
De Mistura, who was in Damascus this week to meet Assad, is due to deliver a report on his mission to the UN Security Council on February 17.
If no solution to the conflict is found, "the only one who takes advantage of it is ISIS Daesh," de Mistura said, using another name for the Islamic State of Iraq and the Levant (ISIL).
ISIL is a "monster waiting for this conflict to take place in order to be able to take advantage", he said.
De Mistura's remarks drew condemnation from the opposition National Coalition as well as from activists on the ground in Syria.
"I think De Mistura is fooling himself if he thinks that Assad is part of the solution," Samir Nashar, coalition member, told AFP news agency by telephone from Istanbul.
"Assad is the problem, not part of the solution."
Nashar disagreed, saying: "If Assad was really interested in fighting Daesh, he would have sent his troops to Raqqa rather than to Douma."
Raqqa is the self-proclaimed capital of ISIL in northern Syria, while Douma is a rebel bastion in the Eastern Ghouta area east of Damascus under a suffocating regime siege for more than a year.
More than 183 people have been killed in near daily bombardment of Douma over the past few weeks, according to the Syrian Observatory for Human Rights, which said 29 children were among the dead.
Separately, Najib Ghadbian, the National Coalition's UN envoy, described the "brutality" of Assad's regime as the root cause of the conflict.
He also warned the US-led alliance fighting ISIL - which has seized swathes of territory in both Syria and Iraq - that its efforts would fail unless world powers get serious about a peace plan for Syria.
Source: Al Jazeera and agencies