Adding to the tensions, Syrians began voting in government-controlled areas in a parliamentary election which is not recognised by the United Nations or by President Bashar al-Assad's main opponents.
The UN-brokered talks in Geneva are aimed at forming a transitional government and a new constitution followed by general elections to end a conflict that has killed more than 270,000 people and displaced half of the country's population.
But Assad's fate remains a major stumbling block.
Russian Foreign Minister Sergei Lavrov stressed the importance of this round of talks, which is scheduled to last about 10 days.
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"The Syrian parties should discuss the new constitution, and how they see the structure that will ensure a peaceful transition towards a new system," he told reporters in Moscow.
A Western diplomat also said "there is more riding on this round" than previous rounds, since de Mistura is looking to tackle the thorny issue of "what does transition away from Assad actually look like".
The main question will be whether the regime delegation will agree to broach the issue of Assad's future, which it until now has insisted is off limits.
De Mistura is "walking in a bit of a minefield," al-Aridi acknowledged.
Complicating matters further, a surge in violence in recent days has threatened a landmark ceasefire agreed in February.
This has piled more pressure on the talks, which follow fruitless attempts in previous years to negotiate an end to the bloodshed.
Power said Moscow had to put pressure on Damascus to "get the regime back with the programme", adding she was "very alarmed" by Syria's plans to launch a Russian-backed counter-offensive in Aleppo, the epicentre of the renewed fighting.