"We ask for direct negotiations... It would save time and be proof of seriousness instead of negotiating in (separate) rooms," Salem al-Meslet, spokesman for the High Negotiations Committee (HNC) umbrella group, told AFP.
During three previous rounds of talks in Geneva last year, the two sides never sat down at the same table, instead leaving UN mediator Staffan de Mistura to shuttle between them.
This time, de Mistura has voiced hope that he will manage to bring the two sides together for direct talks.
But when he was asked about the prospect during a press conference in Geneva Wednesday, the UN envoy was cautious in his response, saying he wanted to talk with the two sides bilaterally first.
The comments came on the eve of a new round of talks in Geneva aimed at finding a political solution to Syria's brutal six-year conflict after a 10-month hiatus.
Separate talks in the Kazakh capital of Astana last week, aimed at shoring up a shaky ceasefire, saw the rebels refuse to talk directly to the regime and did not result in any significant breakthrough.
Meslet pointed out Wednesday that the HNC had also called for direct negotiations before the talks broke off last April.
"We are here to negotiate. . Let's start with direct negotiations," he said, also calling for the talks to focus on the creation of a transitional governing body in Syria.
But that issue is likely to remain a major stumbling block in the talks.
The HNC insists Syrian President Bashar al-Assad must leave office as part of any deal, while Damascus has said the president's future is not open for negotiation.