Syrian opposition negotiators and rebels have temporarily withdrawn from UN-brokered peace talks, vowing to strike back in reaction to the government's alleged ceasefire violations.
While an opposition spokesman says his side has "decided to postpone" the talks that started on Wednesday in Geneva (local time), UN mediator Staffan de Mistura clarified that the delegation has left an opening.
The delegates will suspend their participation in official meetings at UN offices in the Swiss city, he says.
"They told us, however, their intention to remain in Geneva in their hotel and possibly, at my own suggestion, to pursue technical discussions with myself and my team," de Mistura told reporters.
The opposition decided that continuing negotiations despite a lack of progress on humanitarian issues and escalating ceasefire breaches would be "increasing the ordeal of our people," spokesman Ahmed Ramadan says.
De Mistura has been shuttling between the regime and the opposition teams, rather than bringing them to the same table.
US President Barack Obama telephoned Russian President Vladimir Putin to express concern about the status of the Syrian ceasefire and push Putin to pressure the Russian-backed regime of Syrian President Bashar al-Assad to stop attacks against the opposition.
Earlier on Monday, several major Syrian rebel factions vowed to retaliate against government forces for alleged violations of the cessation of hostilities in a further sign that the war-torn country's ceasefire is teetering.
Rebel troops attacked government positions in Latakia province in the north and on the outskirts of western Hama in Syria's central region, according to the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman.
The latest clashes come on the heels of recent skirmishes and a build-up of forces at potential flashpoints.
The opposition accuses the government of using the ceasefire, which was brokered by Russia and the United States on February 27, to gain ground and prepare for future attacks, especially around Aleppo city.
"The cessation of hostilities is still holding in many areas, but the increase in fighting is indeed worrisome," said de Mistura, summing up the United Nation's assessment.
Humanitarian shipments were proceeding too slowly, he said.
In the Geneva talks, the opposition has been demanding a transitional government without the involvement of President Bashar al-Assad, while the government side has ruled out an early departure of the president.
Meanwhile, rebels including Ahrar al-Sham and the al-Qaeda wing al-Nusra Front are under pressure from the Islamic State extremist militia in the northern province of Aleppo.
More than 100,000 people are trapped in a small pocket in northern Syria, amid ongoing clashes between Islamic State and rebels, the aid group Doctors Without Borders (MSF) warned.