Syrian peace talks came close to collapse after 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 said in a letter to rebel fighters that government military advances meant a ceasefire was effectively over and it was calling a postponement in the talks.
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 heavy government air strikes took place in Homs province to the south.
There was "no way" the opposition could resume formal talks amid a military escalation and a worsening humanitarian situation, senior opposition negotiator Mohammad Alloush said.
"We want real negotiations, not farcical ones," he said, adding that the opposition had not decided whether it would continue to meet UN envoy Staffan de Mistura.
Mr de Mistura played down the decision, 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 said he would call on Russia and the United States to meet urgently if the situation did not improve.
The US also urged Russia to use its influence tomake the government stop its attacks immediately.
These threatened not only the partial ceasefire but also the talks in Geneva, a US State Department official said.
Last month Mr 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 US and Russia sponsoring a partial ceasefire since 27 February 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 Mr Assad to leave power.
Mr Assad's opponents say they cannot participate in talks as long as fighting goes on. But they are also under strong pressure not to quit altogether.
