Russia had long sought UN participation in the Sochi conference aimed at advancing toward an end to the six-year war in Syria.
Guterres "is confident that the congress in Sochi will be an important contribution" to reviving the peace talks held under UN auspices in Geneva, said UN spokesman Stephane Dujarric.
The decision came just hours after Syria's main opposition group announced it was boycotting the talks opening in the Black Sea resort of Sochi on Monday.
The boycott by the opposition Syrian Negotiations Commission (SNC) followed two days of UN talks in Vienna that failed to yield progress.
The UN spokesman indicated that Guterres had received assurances that the Sochi conference would not seek to sideline the UN talks.
Guterres was briefed by De Mistura on the outcome of the Vienna talks and has taken into account a statement from Russia that the result of the Sochi conference "would be brought to Geneva as a contribution to the intra-Syrian talks process under the auspices of the United Nations," the spokesman said in a statement.
More than 340,000 people have died in the carnage, millions have fled their homes and the fighting has left the country in ruins.
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