Syrian peace talks have come close to collapse, with the mainstream opposition announcing a "pause" in talks at the United Nations headquarters in Geneva, although it agreed to keep its negotiating team in the city.
The Western-backed opposition High Negotiations Committee (HNC) said in a letter to rebel fighters on Monday that government military advances meant a ceasefire was "effectively over" and it was calling a postponement in the talks.
The rebels, who accuse the government of breaking the ceasefire to try to recapture the northern city of Aleppo, announced an offensive of their own, launching an assault against government forces in Latakia province on the Mediterranean coast.
Opposition fighters made separate advances further east in Hama, while there were heavy government air strikes in Homs province to the south.
Nevertheless, UN envoy Staffan de Mistura played down the decision by the opposition delegation to stay away from his headquarters, saying his team would continue to meet the delegates elsewhere as long as they were still in town.
He acknowledged that violence had become "worrisome", particularly around Aleppo, and that he would call on Moscow and Washington to meet urgently to discuss it if the situation did not improve.
Last month de Mistura convened the first peace talks attended by the warring parties since the conflict began five years ago.
He has come further than any envoy so far in getting President Bashar al-Assad's government and its opponents to negotiate, with the United States and Russia sponsoring a partial ceasefire since February 27 to allow the talks to take place.
So far all sides have committed to some kind of political "transition" that would follow the war, but they still differ fundamentally on what that means, including whether it would require Assad to leave power.
Assad's opponents say they cannot participate in talks as long as fighting goes on, but they are also under strong pressure not to abandon altogether a peace process that has no real alternatives, at a time when Assad's forces have benefited from Russia's support.
"If they walk away, they will be held responsible and it will be difficult to return soon," a Western diplomat said.
De Mistura said the opposition delegation had told him it would "postpone" its "formal participation" in negotiations at the UN's headquarters, known as the Palais.
"It is one way for them to display their displeasure and concern for what they perceive to be a substantial deterioration of the humanitarian situation and a deterioration of the security environment, particularly in Aleppo," he said.
"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."
His team would continue to meet all sides "in the Palais or anywhere else", he added, describing the format of proximity talks, in which the sides negotiate in separate rooms, as "very flexible".
De Mistura acknowledged both sides were "not yielding a comma" on their political demands, but said that was normal in a negotiation.