With Syria peace talks due to start in Geneva on Monday, the Damascus government has said the presidency will not be up for discussion.
The UN-led talks aim to build on a ceasefire deal that has cut violence over the past two weeks, but both sides claim violations by the other.
"The number of violations by the regime exceeded 350 during the past 14 days. All this proves that the regime violated the truce in a way that can't be called violations, but non-commitment," said Mohamad Alloush, High Negotiations Committee.
Ahead of the talks, Syria's Foreign Minister has heaped criticism on the UN envoy, Staffan de Mistura, for saying presidential elections would take place in 18 months time.
"Neither he nor anybody else has the right to talk about presidential elections. The elections are an exclusive right for the Syrian people," said Walid Al-Moualem.
"So what De Mistura said opposes all UN documents that the dialogue, which is to happen, is based on."
The talks will coincide with the fifth anniversary of Syria's war, which has killed more than 250,000 people and sparked a refugee crisis. It has also allowed the expansion of the so-called Daesh militant group.
They are part of the first diplomatic push since the Russian air force intervened in September to support Assad, tilting the war his way and helping Damascus reclaim significant areas in the west.
The ceasefire agreement, brokered by the US and Russia, has been more widely respected than many expected, though fighting has continues on some important fronts, including near the Turkish border.
Copyright © euronews 2019