The opposition High Negotiations Committee 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 Syrian Observatory for Human Rights said the number of civilians killed in flashpoint Aleppo city was one of the highest single tolls since a fragile truce came into force on February 27.
The truce has seen violence drop across parts of Syria, including the northern city of Aleppo, but renewed clashes there in the past 24 hours have seriously strained the truce, the Observatory said.
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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.
"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."
"The negotiations have nearly reached an impasse with the intransigent regime's refusal to negotiate the fate of (President Bashar al-)Assad in the Geneva talks," the member said.
Assad's fate has remained the main sticking point in peace talks, with Syria's opposition clinging onto its call for his ouster since the conflict broke out in 2011.