A Syrian military offensive backed by heavy Russian air strikes has threatened to cut critical rebel supply lines into the northern city of Aleppo.
The offensive comes while both sides in the conflict say peace talks have not started, despite a UN statement to the contrary.
Yesterday UN envoy Staffan de Mistura announced the formal start of the first attempt in two years to negotiate an end to a war that has killed 250,000 people, caused a refugee crisis in the region and Europe and empowered so-called Islamic State militants.
Both opposition and government representatives have since said the talks had not in fact begun and fighting on the ground raged on without constraint.
Mr de Mistura acknowledged that a collapse of the Geneva talks was always possible.
"If there is a failure this time after we tried twice at conferences in Geneva, for Syria there will be no more hope. We must absolutely try to ensure that there is no failure," he told Swiss television RTS.
The opposition cancelled a meeting with him this afternoon, and issued a statement condemning "a massive acceleration of Russian and regime military aggression on Aleppo and Homs", calling it a threat to the political process.
Rebels described the assault north of Aleppo as the most intense yet.
One commander said opposition-held areas of the divided 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.
The main Syrian opposition council said after meeting Mr de Mistura it had not, and would not, negotiate unless the government stopped bombarding civilian areas, lifted blockades on besieged towns and released detainees.
Conditions are dire in a number of areas under siege by both sides, with many close to starvation.
However, the Syrian Red Crescent delivered 14 truckloads of aid to the town of al-Tal north of Damascus, in an area surrounded by forces allied to the government.
The head of the Syrian government delegation also denied talks had started after discussions with Mr de Mistura today.
Bashar al-Ja'afari said after two and a half hours of talks the envoy had yet to provide an agenda or list of opposition participants.
"The formalities are not yet ready," he told reporters at the UN office in Geneva.
He also said that if the opposition "really cared" about the lives of Syrians it should condemn the killing of more than 60 people on Sunday by IS bombers in a neighbourhood that is home to the country's holiest Shia shrine.
A UN source said Mr de Mistura had promised to present an opposition delegation list by tomorrow.
Its makeup is subject to fierce disagreements among the regional and global powers that have been drawn into the conflict.
The refugee crisis and spread of the jihadist IS group through large areas of Syria, and from there to Iraq, has injected a new urgency to resolve the five-year-old war.
The chances of success, always very slim, appear to be receding as the government, supported by Russian air strikes, advances against rebels, some of them US-backed.
US Secretary of State John Kerry called on Russia 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 IS.
