Staffan de Mistura also said he intended to convene further talks in August and September.
De Mistura added meetings on constitutional and legal issues will continue alongside the formal sessions.
The last round of UN-sponsored peace talks finished on May 19th with De Mistura telling the UN Security Council that "important gaps remain between the parties on major issues," not least the fate of President Bashar al-Assad.
The opposition insists Assad must leave power as part of any peace deal but the government has rejected the demand.
The talks are aimed at ending a war that has killed more than 320,000 people since 2011 and laid waste to much of the country's infrastructure.
Following the last round De Mistura said time constraints had stymied progress on key topics, including a mooted new constitution and the organisation of elections.
At the last talks, De Mistura said the Geneva participants were not seeking to draft a new constitution, but rather "laying foundations for the time when the Syrians can do that."
Parallel negotiations have been held in the Kazakh capital Astana, where Turkey and Assad allies Russia and Iran agreed in May to establish four safe zones across Syria to ban flights and ensure aid drops.
But the plan has stalled amid disagreement over details such as which countries should send in forces to police the zones.
A new round of Astana talks had been scheduled for the coming days but has been postponed indefinitely.