Syrian peace talks have broken down after the UN mediator said the opposition had suspended its formal participation in the indirect discussions with President Bashar Assad's government in protest over "worrisome" new violence.
Staffan de Mistura, the UN envoy for Syria, said he will press on with the talks despite the suspension by the opposition High Negotiations Committee.
The HNC said it would no longer attend meetings at the UN office in Geneva that has hosted weeks of on-off peace talks aimed at ending the country's devastating five-year war.
The HNC delegation will remain at its hotel in Geneva, and Mr de Mistura said he will continue "technical" discussions with its envoys by phone or off-site in hopes of firming up a blueprint for a political transition in Syria.
He said he would "take stock" of progress towards that goal on Friday.
Mr de Mistura has repeatedly said that political transition - as called for under a UN Security Council resolution - is "the mother of all issues".
On Monday, he acknowledged a "gap" between the two sides on that issue: the HNC wants no role for Assad in any transitional government, and Assad's envoys have proposed a "broad-based government".
"Both of them (are) claiming this is the road to political transition," he said, adding that the two sides agreed on the need for such a transition, but had differences of "interpretation" about what that meant.
The suspension came hours after Syrian opposition fighters launched a new offensive against government forces in a number of north-western areas.
Rebel groups said attacks in rural parts of Latakia province, a government stronghold, were in retaliation for violations of a US and Russian-brokered ceasefire.
Mr de Mistura said the new fighting in parts of Syria, especially near Aleppo, was "particularly worrisome", and he expected that the US and Russia - who are leading oversight of the truce - will hold a special meeting about it "if this trend continues".
In a statement, the Kremlin said Russian President Vladimir Putin and US President Barack Obama discussed the situation in Syria and agreed to increase co-ordination between the special services and militaries of the two countries.
The HNC's chief negotiator, Mohammed Alloush, said the fragile ceasefire that started in late February "has effectively been ended by the regime".
He said government forces carried out 70 air raids on Sunday, Iran had sent in two new fighting groups to help reinforce Assad's troops, and Russia had supplied Syrian soldiers with weaponry.
"All this intervention gives a clear indication that the solution in Syria, with the presence of this regime, has become shut - or we have hit a wall," he said.
The rebels said the offensive came in response to government attacks against refugee camps and residential areas.
Different rebel factions shared videos of their fighters lobbing rockets at government positions in the Jabal al-Akrad area, close to the rebel-held northwestern province of Idlib.
They claimed on social media to have gained ground.
The Britain-based Syrian Observatory for Human Rights confirmed the rebels seized control of at least two areas and said al Qaida's branch in Syria, the Nusra Front, is taking part in the fighting.
Neither the Nusra Front nor the Islamic State group is included in the ceasefire.