"There have to be two, credible delegations to negotiate," Eliasson told reporters in Geneva, the planned venue of the talks.
"We're working very hard for a meeting as soon as possible. We're in contact with the parties, and with the Security Council members, that are involved. But we hope very much that the meeting will take place, and soon," he said.
Amid growing speculation that the talks could be held from June 10, Eliasson declined to be drawn on potential dates but confirmed that the target was still next month.
Last Friday, UN chief Ban Ki-moon and Russia agreed that a peace conference should be held as soon as possible, even as Security Council member Moscow defied growing global pressure over its arms supplies to the Damascus regime.
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The talks are meant to include both the fiercest rebels and members of the regime -- a problem given some opposition members' refusal to recognise President Bashar al-Assad as a negotiating partner.
That plan's measures include a ceasefire and a shift to a transitional government in Syria.
US President Barack Obama said last week that he will continue to press for Assad to leave power even if this is no longer a precondition of the Geneva talks.
The goal of holding talks was agreed during a May 7 visit to Moscow by US Secretary of State John Kerry.