Fighting in Aleppo killed at least 22 civilians on Sunday, as the opposition delegation threatened to quit Syria peace talks in Geneva if there is no progress on a political transition.
The opposition High Negotiations Committee (HNC) said the indirect negotiations could collapse if Syria's regime refuses to compromise on political and humanitarian issues.
"We might suspend (our participation in) the talks if things carry on this way, and then there will be no prospect for any political solution," HNC member Abdulhakim Bashar told AFP.
The opposition's chief negotiator also said there could be "no compromise" on President Bashar Al-Assad's ouster, and called for renewed attacks on regime forces despite a fragile ceasefire.
The comments come a day after a spokesman for HNC said the opposition had rejected a proposal from UN envoy Staffan de Mistura that would have kept Assad as president during a political transition, with three deputies of his opponents' choosing.
Meanwhile, the Syrian Observatory for Human Rights said Sunday the number of civilians killed in flashpoint Aleppo city was one of the highest single tolls since the truce began on February 27.
The truce has seen violence drop across parts of Syria, including the northern city, but renewed clashes there in the past 24 hours have seriously strained the truce, the Observatory said.
At least six civilians were killed and eight wounded in regime air strikes on rebel-held eastern parts of the city on Saturday.
And a barrage of rockets and sniper fire by opposition groups onto government-controlled western districts killed 16 civilians, including 10 children and two women.
Rebel groups fired more rockets at western areas of Aleppo city late Sunday, but there was no immediate information on new casualties.
"There's a clear escalation. This was the bloodiest incident in Aleppo and its province" since the ceasefire began, Observatory head Rami Abdel Rahman said.
"This escalation directly threatens the truce."
The HNC has questioned the regime's commitment to a political solution to Syria's five-year war, particularly in the wake of the renewed violence in Aleppo.
"The humanitarian situation is continually deteriorating, the issue of the detainees has not seen any progress, the ceasefire has almost collapsed, and now there is an attack on Aleppo from three sides," Bashar said in Switzerland.
"Given these factors, we are reviewing everything, and we will continue our meetings today (Sunday) so that tomorrow we can decide what to do," he added.
A second member of the HNC delegation, speaking anonymously, said the talks were nearly at "an impasse".
"The negotiations have nearly reached an impasse with the intransigent regime's refusal to negotiate the fate of Assad in the Geneva talks," the member told AFP.
"There can be no compromise on the issue of Bashar al-Assad… For us, it's a closed book — you cannot trade an entire people for one man," opposition chief negotiator Mohammed Alloush told the news agency in Geneva.
Earlier, Alloush called for renewed attacks on regime forces, despite the shaky truce.
"Don't trust the regime and don't wait for their pity," tweeted Alloush, a leading political figure in the Jaish al-Islam (Army of Islam) rebel group.
"Strike them at their necks (kill them). Strike them everywhere," he added.
A fellow opposition figure said Alloush's hawkish statement did not represent the HNC's position.
Alloush himself later told AFP that he had been calling on rebel groups to defend themselves against regime truce violations.
Since the conflict erupted in 2011, half of Syria's population has been displaced — including five million who have fled to neighboring states. More than 270,000 people have been killed.
AFP contributed to this report.