Syrian peace talks have been “temporarily halted” because of a major advance by Assad forces against rebel forces north of Aleppo.
The government advance - which was backed by Russian air strikes - has choked opposition supply lines from Turkey to the city and disrupted humanitarian aid, a US State Department official said.
The advance saw the army break a three year siege of two government controlled towns in the area - Nubul and Zahraa - which were previously surrounded by opposition territory.
Speaking at a news briefing, the state department spokesman John Kirby said the talks - which officially began two days ago in Geneva, Switzerland - did get under way before the new Syrian army assault on Wednesday.
He said the assault was mainly focused on opposition forces and urged Moscow to focus on routing out Isis.
UN envoy Staffan de Mistura, who suspended the talks, said they will resume on 25th February, adding “this is not the end, and it is not the failure of the talks” and insisted both sides were “interested in [getting] the political process started”.
But the Syrian government’s UN ambassador Bashar al-Ja’afari accused the opposition of pulling out of the talks because it was losing the fight on the ground.
He said: "The military developments on the ground were crucial. Notice that the [opposition] decision of withdrawal came after opening the road to Nubul and Zahraa".
Nubul and Zahra have been blockaded by rebel groups for around three years, with the army occasionally dropping food and other aid by air.
In pictures: Russian air strikes in Syria
In pictures: Russian air strikes in Syria
Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria
The aftermath of Russian airstrike in Talbiseh, Syria
Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes
Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit
Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria
Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps
Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed
A release from the Russian defence ministry purportedly showing targets in Syria being hit
A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria
Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria
Their capture marks a major victory for government forces, which have made significant advances in Aleppo province in the past few days.
Basma Kodmani, a member of the opposition's negotiating team, described the offensive as a "horrible development," saying it sends the message that "there is nothing to negotiate. Just go home."
Mr De Mistura said both sides had expressed concerns about the talks, with the government wanting to address "procedural issues before talking about [the] humanitarian side."
But he said "the UN cannot allow simply procedural matters to become more important than the result for the humanitarian situation for the Syrian people, who have been waiting for us to deliver... something concrete for them."
The talks are aimed at ending the five year long civil war between the Assad government and rebel groups which has killed more than 250,000 people and displaced 11m more.
Isis and fellow Islamist group Jabhat al-Nusra have not been included.
Additional reporting by agenciesReuse content