The UN's special envoy on Tuesday warned that all hope to resolve Syria's civil war would be lost if the latest attempt at peace talks failed, after the opposition said Russian airstrikes threatened to derail the discussions before they had begun.
The main opposition umbrella group attending the talks said Russia's "unprecedented" bombardment near Aleppo -- 270 raids since Monday morning, according to monitors -- threatened to scupper efforts to end the almost five-year conflict.
"Since last night a big massacre is taking place in Syria and nobody is doing anything. Nobody is saying anything, the international community is completely blind," said Salem al-Meslet from the High Negotiations Committee (HNC), according to the AFP news agency.
On Monday, UN special envoy Staffan de Mistura declared that indirect talks between the government and the opposition had officially begun in Switzerland, saying he hoped to "achieve something" by February 11.
But as cracks began to emerge on Tuesday he warned they were the last chance to bring about an end to a conflict that has left 260,000 people dead and forced more than half of Syria's population to flee their homes.
"If there is a failure this time, after two previous meetings in Geneva on Syria, then all hope will be lost," he told Swiss TV channel Radio Television Suisse.
The chief negotiator for Syria's government, Bashar al-Jaafari, had earlier cast doubt on the gravity of the talks, saying they were still "in a preparatory phase", the opposition had not named its negotiating team and there was no agenda.
The HNC later cancelled a meeting with the UN envoy scheduled for Tuesday afternoon, with member Farah Atassi saying that "at this moment, there is no reason to repeat ourselves with de Mistura".
The group has demanded the regime allow humanitarian access to besieged towns, stop bombing civilians and release thousands of prisoners -- some of them children -- languishing in regime jails.
Earlier on Tuesday, Syria's Red Crescent delivered aid to a town under government siege outside the capital Damascus. Of significant concern is the town of Madaya, which is under government siege as well, and where thousands are said to be starving.
The HNC also expressed outrage at the regime offensive, backed by Russian jets and allied militants, that allowed government forces to edge closer to breaking a long-running rebel siege on two government-held Shiite villages in Aleppo province.
The Syrian Observatory for Human Rights, a monitoring group, said at least 18 civilians had been killed in the raids on Tuesday, including five women, three children and two emergency workers.
"We have never seen things like this since the beginning of the revolution," HNC spokeswoman Basma Kodmani said, according to AFP, calling the air raids "unprecedented".
"The regime's and Russia's actions gravely threaten the political process at this early stage," fellow HNC member Atassi said.
In a November meeting in Vienna, world powers agreed on an ambitious road map that foresees six months of intra-Syrian talks, leading to a new constitution and free elections within 18 months.
But it left unresolved the future of embattled Syrian President Bashar Al-Assad.
The conflict has dragged in a range of international players, from Iran, Turkey and the Gulf states -- the latter two on the opposition side -- to Western nations and, since late September, Russia.
The chaos has also fuelled the rise of the Islamic State (ISIS) group, which has overrun swathes of Syria and Iraq and staged a series of deadly attacks across the globe, including in Paris in November.
The group claimed responsibility for multiple blasts on Sunday on a revered Shiite shrine south of Damascus that killed at least 70 people.
In Geneva, Assad's government has been objecting to the inclusion in the Saudi-backed HNC of certain rebels it denounces as "terrorists", a stance supported by Moscow and by Iran, Riyadh's arch rival.
One of these is Mohammed Alloush, a member of the powerful Army of Islam armed rebel group who arrived in Geneva late Monday to act as the HNC's chief negotiator.
AFP contributed to this report.