"The situation is fragile, success is not guaranteed but progress is visible," de Mistura told reporters.
De Mistura plans to start a round of peace talks on March 9, but the format for the indirect talks is flexible and some parties could turn up days later, he said, Reuters reported.
The ceasefire—sponsored by the US and Russia—is meant to last for two weeks. It represents the biggest effort in years to calm violence in a war that has claimed more than 260,000 lives and displaced millions.