A Syrian military offensive threatened critical rebel supply lines into the northern city of Aleppo on Tuesday and Damascus echoed its opponents in contradicting a U.N. envoy's assertion peace talks had begun.
U.N. envoy Staffan De Mistura announced the formal start on Monday 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 of Iraq and Syria (ISIS) 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.
The U.N. envoy said on Tuesday a complete failure of the Syria peace talks is "always possible", but all efforts must be made to resolve the devastating conflict.
Asked whether the negotiations in Geneva could be a complete failure, de Mistura replied: "It is always possible, particularly after five years of a horrible war, where the sides hate each other, where there is a huge lack of confidence.
"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."
A rebel commander told Reuters he was deploying reinforcements including U.S.-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 ISIS bombers in a neighborhood that is home to the country's holiest Shiite shrine.
A U.N. 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 ISIS 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 U.S.-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 Sept. 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 Shiite 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 10 km 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 center, is divided between areas controlled separately by the government and opposition.
The rebel commander said the Russian air force was mounting heavy air strikes in the area.
"We sent new fighters this morning, we sent heavier equipment there. It seems it will be a decisive battle in the north God willing," said Ahmed al Seoud, head of a Free Syrian Army group known as Division 13. "We sent TOW missile platforms. We sent everything there," he told Reuters.
U.S.-made TOW missiles, or guided anti-tank missiles, are the most potent weapon in the rebel arsenal and have been supplied to vetted rebel groups as part of a programme of military support overseen by the Central Intelligence Agency.
A correspondent with the pro-government Al Mayadeen TV embedded with the army said there had been 150 air strikes in the last two days. A tank and armored vehicle were shown driving through a road in a largely destroyed village.
The sound of jets and crackle of automatic gunfire could be heard during a broadcast by the pro-opposition Orient TV.
The Russian intervention has reversed the course of the war for Damascus, which suffered a series of major defeats to rebels in western Syria last year before Moscow deployed its air force as part of an alliance with Iran.
In an interview with Reuters, British Foreign Secretary Philip Hammond said Russian President Vladimir Putin was undermining international efforts to end the war by bombing opponents of ISIS in an attempt to bolster Assad.
Opposition wary of envoy
"The Russians say let's talk, and then they talk and they talk and they talk. The problem with the Russians is while they are talking they are bombing, and they are supporting Assad," Hammond said.
Russia's Foreign Ministry said Hammond was spreading "dangerous disinformation", while the Kremlin said his statements could not be taken seriously.
Western states opposed to Assad, including the United States and Britain, piled pressure on the opposition to attend the Geneva talks which have been beset by problems including a row over who should be invited to negotiate with Damascus.
In the latest downbeat opposition assessment, lead opposition negotiator Mohamad Alloush said he was not optimistic. "Nothing has changed in the situation on the ground so as long as the situation is like this we are not optimistic," he told reporters. "There are no good intentions from the regime's side to reach a solution."
He was speaking minutes before the government delegation arrived at U.N. headquarters in Geneva to meet De Mistura to discuss a proposal on humanitarian issues.
De Mistura said on Monday the responsibility of agreeing ceasefires across Syria lay with major powers and that his remit was only to hold talks on a U.N. resolution on elections, governance and a new constitution.
All previous diplomatic efforts have failed to stop the war.
Complicating the efforts, opposition mistrust of de Mistura is running deep: "The fact is the opposition have a lot of distrust in de Mistura. They have become extremely wary," a Western diplomat said.