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.
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.
And the HNC 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.
It 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.
- 'Unprecedented' bombings -
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, 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.
US Secretary of State John Kerry weighed in Tuesday, urging the Syrian opposition to remain in peace talks despite the Russian bombing, adding that he was "extraordinarily sympathetic" to their difficult situation.
"But the agreement at the United Nations and the agreement in Vienna is that when the political dialogue begins there will be a ceasefire. So the hope, the expectation is that it shouldn't take long and we're not requiring people to sit at the table for months," Kerry said.
Russia said it was willing to coordinate efforts toward a Syria ceasefire with the US, according to Russian news agency Interfax.
It also quoted deputy foreign minister Gennady Gatilov saying de Mistura would again meet the opposition negotiators "tomorrow or the day after tomorrow" and they "will announce the make-up of the delegation that will participate in talks".
- Last chance talks -
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 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 extremist Sunni Muslim 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.