"All invitees who had already attended the previous round of talks in February 2017 have confirmed their participation", UN spokeswoman Alessandra Vellucci told reporters.
She said UN mediator Staffan de Mistura was currently visiting key powers shaping the conflict -- including Saudi Arabia, Russia and Turkey -- before returning to Geneva to lead the talks.
As de Mistura "completes these consultations, Deputy Special Envoy Ramzy (Ezzeldin Ramzy) will engage the parties bilaterally on 23 March in Geneva to prepare the round ahead," Vellucci said.
The participation of all rebel groups could bolster the UN peace process, after some opposition camps refused to attend separate negotiations in Kazakhstan last week to protest violations of a shaky ceasefire by the Syrian regime.
The parallel series of talks in Astana is aimed at bolstering the truce, in view of supporting the UN negotiations.
Damascus has been the scene of heavy fighting in recent days, with rebels and jihadists making large incursions into the east of the Syrian capital.
Syrian President Bashar al-Assad's regime has responded with air strikes on rebel positions.