The second day of exploratory talks on a peace process in Syria have ended in Geneva, with opposition delegates calling off a meeting with the UN special envoy, Staffan de Mistura.
A Syrian military offensive backed by heavy Russian air strikes threatened to cut critical rebel supply lines into the northern city of Aleppo, as peace talks in Switzerland appeared to be in jeopardy on Wednesday.
The government attack north of Aleppo that began in recent days is its first major offensive there since Russian air strikes began on September 30.
Rebels described the assault as the most intense yet. One commander said opposition-held areas of Syria's largest city were at risk of being encircled entirely by the government and allied militia, appealing to foreign states that back the rebels to send more weapons.
Chances of achieving a ceasefire at talks in Geneva appear to be receding as the government, supported by Russian air power, advances against rebels, some of them US-backed.
The refugee crisis and spread of the Islamic State of Iraq and the Levant (ISIL) through large areas of Syria, and from there to Iraq, had injected a new urgency to resolve the five-year-old Syria war.
US Secretary of State John Kerry called on Moscow to stop the bombing during the peace process. "We are beginning the talks, we are at the table and we expect a ceasefire," he said after a meeting in Rome of countries opposed to ISIL.