The two days of negotiations in Vienna come after eight previous rounds in Geneva, the last one in December, that failed to get the different parties even to talk to each other.
They stumbled over the fate of Syrian President Bashar al-Assad, who has taken the upper hand in the conflict since Russia began backing him militarily in 2015.
Representatives from Assad’s government have refused to meet the opposition directly until they drop demands that he leave office.
A UN spokeswoman said the Syrian government delegation was due to meet with UN envoy Staffan de Mistura at 11:30 am (1030 GMT).
The main opposition group, the Syrian Negotiations Commission (SNC), said it would sit down for separate talks at 4 pm.
De Mistura said on Wednesday that the negotiations in Vienna involving “full delegations” of the opposition and government come at a “very, very critical moment”.
Nasr al-Hariri from the SNC said that the next two days would be “a real test for all the sides”.
French Foreign Minister Jean-Yves Le Drian said meanwhile in Paris that the talks are the “last hope” for reaching a political solution.
Le Drian also highlighted a “considerable worsening of the humanitarian situation” in Afrin, as well as in Idlib and in Eastern Ghouta.
In the Afrin enclave in northern Syria, Turkey on Saturday launched an offensive alongside Syrian rebels to oust the Kurdish People’s Protection Units (YPG) militia.
The Turkish intervention — its second in a conflict that has drawn in multiple world powers — has heightened tensions with Ankara’s NATO ally the United States, which has backed the Kurdish militants in their battle against the Islamic State group.
US President Donald Trump clashed with his Turkish opposite number Recep Tayyip Erdogan on Wednesday, with Trump urging Erdogan to “de-escalate” the fighting in Afrin.
Erdogan insisted that his Operation Olive Branch was a legal defence of Turkey’s national security, as Ankara views the Kurdish YPG fighters as a Syrian offshoot of the outlawed Kurdistan Workers’ Party (PKK).
– Suspected chemical attack –
Meanwhile Russian-backed Syrian troops have been carrying out an offensive since late 2017 in the northwest region of Idlib, the last province still fully outside government control.
This has forced tens of thousands of people to flee. On Sunday the Syrian army said it had captured the vital Abu Duhur military airport.
In Eastern Ghouta, a besieged rebel stronghold outside Damascus, at least 21 people suffered breathing difficulties on Monday, a monitor said, in a suspected chemical weapons attack by the regime that prompted a sharp US warning to Russia to rein in its ally Assad.
– Parallel peace talks –
Russian-backed Syrian forces have also dealt severe blows to IS, whose self-proclaimed “caliphate” in Iraq and Syria has largely collapsed.
The Vienna talks come ahead of a “Congress of National Dialogue” peace conference in the Russian Black Sea resort of Sochi backed by Russia, Iran and Turkey next Tuesday.
The three key regional players have been sponsoring parallel peace talks since the start of last year that have looked to still the fighting.
Sochi is part of a broader Russian push to start hammering out a path to a political solution to end the war, sparking concerns that the Kremlin is looking to sideline the UN.
The focus in Sochi will be on hammering out a new constitution, according to the opposition, something that de Mistura also wants discussed in Vienna.
While Assad’s government has said it will go to Sochi, the SNC has not yet decided, even after a recent visit to Moscow.
Expert Firas Modad from IHS Markit said that progress in Vienna “seems unlikely”, and that Moscow, Tehran and Ankara “are the ones driving the negotiation process”.
“Assad is focused on total victory, with no interest in compromise,” he told AFP.