Key Syrian rebel groups vowed on Monday to strike back against alleged regime ceasefire violations, casting doubt on the future of fragile peace talks due to resume in Geneva.
A truce agreed in February dramatically curtailed violence across much of Syria, but fighting has surged in recent days around second city Aleppo, causing tens of thousands to flee.
"After the increase of violations by regime forces that included targeting displaced people and continuous bombing of residential neighbourhoods, we declare the start of the battle in response," said a statement signed by 10 armed rebel groups.
In Geneva, where regime and opposition delegations were set to restart indirect negotiations, Syria's main rebel delegation warned that renewed fighting could scupper peace talks.
"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 landmark ceasefire agreed between the United States and Russia took effect on February 27, raising hopes that a lasting deal could be struck in Geneva to end Syria's five-year civil war.
But fighting has spread in the last week around Aleppo, leading the HNC to question President Bashar al-Assad's commitment to a political solution to a conflict that has displaced half of the population and killed more than 270,000 people.
"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.
The Syrian Observatory for Human Rights monitoring group said 22 civilians were killed over the weekend in Aleppo city -- one of the highest single tolls since the truce began.
At least six civilians were killed and eight wounded in regime air strikes on rebel-held eastern parts of the city on Saturday.
A barrage of rockets and sniper fire by opposition groups onto government-controlled western districts killed 16 civilians, including 10 children and two women.
And rebel groups fired more rockets at western areas of Aleppo city late Sunday, but there was no immediate information on new casualties.
- 'Strike them everywhere' -
"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.
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"This escalation directly threatens the truce."
Among the armed groups who signed the Monday statement is Jaish al-Islam, the most important opposition faction in East Ghouta, a key rebel-held town east of Damascus.
Mohammed Alloush, the HNC's chief negotiator in Geneva, is a senior member of Jaish al-Islam who on Sunday called on rebel groups to "strike" regime positions.
"Don't trust the regime and don't wait for their pity," he posted on Twitter. "Strike them at their necks (kill them). Strike them everywhere."
A fellow opposition figure said Alloush's hawkish statement did not represent the HNC's position and Alloush clarified that he was calling on rebels to defend themselves from attacks.
Areas controlled by the Islamic State group, Al-Qaeda's Syria affiliate Al-Nusra Front, and other jihadists are exempt from the ceasefire, but renewed Aleppo clashes are straining the truce as other rebel groups are dragged into the fighting.
IS has seized fresh territory from rebel groups in the north, threatening the key opposition town of Azaz, just eight kilometres (five miles) south of the Turkish border.
The jihadist onslaught has forced 30,000 Syrians to flee, and tens of thousands more are at risk of displacement.
- Assad's fate 'red line' -
In addition to Jaish al-Islam, the rebel statement was signed by the powerful Islamist Ahrar al-Sham faction, which is allied to Al-Nusra and fights alongside it around Aleppo and in neighbouring Idlib province.
UN mediator Staffan de Mistura is expected to sit down with the Damascus government in Geneva before meeting the opposition delegation later Monday.
The fate of Assad remains a major bone of contention, with Syria's opposition clinging onto its call for his ouster since the conflict began in 2011.
Alloush said there could be "no compromise" on Assad's removal but the regime has called that a "red line".
The peace plan outlined by De Mistura and backed by world powers envisions a political transition, a new constitution, and presidential and parliamentary elections by September 2017.
But Syria's government hosted its own parliamentary elections last week only in government-held areas, which Assad's ruling Baath party easily won. The opposition denounced the election as a "farce".