Staffan de Mistura told the Security Council that "important gaps remain between the parties on major issues" following the sixth round in Geneva held last week.
The Syrian government and the opposition took part in four days of indirect talks aimed at ending the war that has killed more than 320,000 people since 2011.
De Mistura said all sides were ready to return to the table for a new round of talks "which we intend to target sometime in June."
Hopes for a breakthrough were dim when the negotiations opened last week, with the fate of President Bashar al-Assad still a major stumbling block.
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