Speaking to reporters after meeting delegates of the opposition bloc in Geneva on Monday, de Mistura said: "We are starting officially the Geneva talks... The discussions are starting."
The Swedish-Italian diplomat said he expected the talks to be "complicated and difficult," but that Syria's people deserved to "see something concrete, apart from a long, painful negotiation."
"The first immediate objective is to make sure the talks continue and that everyone is on board," de Mistura said, AL Jazeera reported.
He could not comment on how long he expected the first round of talks to last, but said he hoped negotiations "achieve something" by February 11.
"The duration of the negotiations depends on the willingness and the determination of both sides," he added.
The talks are part of the biggest push to date to chart a way out of the tangled Syrian war that has killed more than 260,000 people and forced millions from their homes since the violence began in March 2011.
The urgency to find a solution was brought home on Sunday when attacks claimed by Daesh killed 71 people near a revered Shiite shrine outside the capital Damascus, according to monitors.