Riad Naasan Agha, of the Riyadh-based High Negotiations Committee, said that the talks which are set to resume on April 11 in Geneva must focus on the future of the Syrian leader.
"If negotiations did not address the fate of Assad, it would be a waste of time and doomed to failure," he said late Tuesday at a forum hosted by Al-Jazeera in Qatar.
The UN has said the upcoming round of talks will focus on plans for a political transition to lead Syria out of five years of brutal civil war.
Agha said that he was not hopeful the talks would produce a positive outcome as negotiations on forming a transitional government were almost at a "dead-end".
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Negotiators from the regime are expected to attend the talks but only after the completion of parliamentary elections in the country on April 13.
The previous round of talks broke up on March 24, without making any concrete advances towards a political solution to the devastating war.
The opposition wants Assad to leave power before any transitional government is agreed, but the regime says his future is not up for discussion.
The United Nations envoy on Syria, Staffan de Mistura, on Tuesday met Russian Foreign Minister Sergei Lavrov, whose country is a key backer of Assad, in Moscow as he prepares for the Geneva talks.
De Mistura is also expected to visit several other countries ahead of the talks, including Iran, Turkey and Saudi Arabia.