By Tom Perry, Suleiman Al-Khalidi and John Irish
A Syrian military offensive has threatened critical rebel supply lines into the northern city of Aleppo with the government echoing its opponents by contradicting the assertion of a UN envoy that peace talks had begun.
UN envoy Staffan De Mistura announced the formal start on Monday (local time) 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 Islamic State militants.
But both opposition and government representatives have since said the talks have not in fact begun and fighting on the ground raged on without constraint.
A rebel commander told Reuters he was deploying reinforcements including US-made anti-tank missiles to the Aleppo frontline for what he described as a "decisive battle".
The main Syrian opposition council said after meeting De Mistura on Monday it had not and would not negotiate unless the government stopped bombarding civilian areas, lifted blockades and released detainees.
The head of the Syrian government delegation also denied talks had started after discussions with De Mistura on Tuesday.
Bashar al-Ja'afari said after two-and-a-half hours of talks that the envoy had yet to provide an agenda or list of opposition participants. "The formalities are not yet ready," he told reporters at the United Nations 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 Islamic State bombers in a neighbourhood that is home to the country's holiest Shi'ite shrine.
A UN source said de Mistura had promised to present an opposition delegation list by Wednesday. 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 Islamic State through large areas of Syria, and from there to Iraq, has injected a new urgency to resolve the five-year-old Syria war.
But the chances of success, always very slim, appear to be receding ever more as the government, supported by Russian air strikes, advances against rebels, some of them US-backed, in several parts of western Syria where the country's main cities are located.
The attack north of Aleppo that began in recent days is the first major government offensive there since the start of Russian air strikes on September 30.
The area is strategic to both sides. Its safeguards a rebel supply route from Turkey into opposition-held parts of the city and stands between government-held parts of western Aleppo and the Shi'ite villages of Nubul and al-Zahraa which are loyal to Damascus.
The Syrian Observatory for Human Rights, which reports the war using a network of sources on the ground, said the army and allied fighters captured more areas to the northwest of Aleppo on Tuesday.
Advancing government forces seized the village of Hardatnin some 10km northwest of Aleppo, the Observatory said, building on gains the previous day. Syrian state media also reported the advance.
Aleppo, once Syria's biggest city and commercial centre, is divided between areas controlled separately by the government and opposition.