The UN envoy to Syria, Staffan de Mistura, has described fresh peace talks in Geneva as a “moment of truth”.
Speaking before the negotiations, Mr de Mistura said “the mother of all issues” was political transition and the only alternative was a return to war.
But neither Syria’s government nor the opposition appear ready to compromise over the future of President Assad.
Diplomats are hoping to build on the fragile and partial truce, which came into effect at the end of February.
The UN-led talks represent the first serious diplomatic intervention since Russia began air strikes in September.
Syrian Foreign Minister Walid Muallem on Saturday ruled out any discussion on the future of President Assad.
But the main Syrian opposition alliance says Mr Assad has to go.
US Secretary of State John Kerry responded by accusing Damascus of “trying to disrupt the process”.