The government of Syrian president Bashar Assad has effectively "buried" the ceasefire, the head of a Turkey-based Syrian opposition group said, warning that already-troubled peace negotiations may collapse completely.
Anas Abda, the leader of the Syrian National Coalition (SNC), said there will be no quick return to peace talks in Geneva if the current situation continues.
A ceasefire agreement in place since February 27 which greatly reduced levels of violence in Syria's civil war has all but collapsed amid fierce fighting in Syria's north in the past week.
The opposition delegation to the talks in Geneva suspended its participation in protest although it kept a small technical team there.
It later said that government air strikes targeted two opposition-held towns in north-western Syria, killing 44 civilians, and said such attacks validated its decision.
"The regime buried the truce yesterday," Mr Abda said, speaking at a press conference in Turkey.
He claimed that there have been more than 2,100 violations of the truce by government forces in the past 53 days.
"There will be no quick return to negotiations if the current situation continues," he added.
The SNC is part of the Higher Negotiations Committee (HNC), the Syrian opposition coalition negotiating in Geneva.
Mr Abda's comments echoed those of HNC chief Riad Hijab, who said there can be no solution in Syria with Assad in power and called for international monitors to observe a ceasefire in Syria.
He vowed to fight "even with stones" to depose Assad, shifting sharply to a tone of conflict over conciliation.
The calculated gamble to jeopardise what diplomats have called the best chance in years to bring a diplomatic end to Syria's five-year war came amid the opposition group's growing frustration over unproductive peace talks and hundreds of government ceasefire violations in recent weeks.
The opposition coalition accuses the government of preparing an assault on the city of Aleppo, ignoring its demands for the release of thousands of detainees, and rejecting or avoiding requests for UN-led humanitarian aid shipments in recent weeks.
The opposition says those are signs of bad faith by Assad's side and accuses it of stalling for time in the Geneva talks.
Opposition activists and a media outlet associated with militants said the Islamic State group, which is excluded from the ceasefire agreement, has seized control of a government-controlled neighbourhood in Deir el-Zour in eastern Syria, expanding its presence in the province.
The Britain-based Syrian Observatory for Human Rights said the group's militants clashed heavily overnight with government troops and allied forces, advancing in the city's industrial district. The observatory said five air strikes hit the neighbourhood, but there was no immediate word of casualties or who was behind the strikes.
The IS-linked Aamaq news agency said IS has expelled government troops from the district. The militant group now edges closer to a much-coveted military air base.
IS recently lost control to the government of the city of Palmyra, but has also repelled attacks from other rebel groups in the north.