A US-Russia-brokered ceasefire in Syria does not apply to terrorist groups such as Daesh and al-Nusra Front.
"Despite of several incidents cessation of hostilities is still holding…we need to make sure that it continues in spirit and substance," de Mistura told reporters following a meeting with the Syrian opposition's High Negotiations Committee.
"Tomorrow we will be actively raising the issue of those incidents at the meeting of the task force," he added.
HNC head Asaad Al-Zoubi told reporters minutes earlier that Moscow had yet to define its position regarding a political transition in Syria.
"I have to correct a little bit the perception of who proceeded me," de Mistura said, commenting on Al-Zoubi's statements at a press conference.
"They all [Moscow, Tehran, Amman, Damascus] indicated interest and support, actually, in the progress of a political discussion aiming at a political transition. And with all of them I'm being very clear that what we aim at is actually an agenda which is based on political transition with the issue of governance and constitution, as in Resolution 2254," he added.
"None of them, actually, expressed any doubt that that is the priority and the agenda," the diplomat underlined.