"I heard today from the HNC their intention to postpone their formal participation in the Palais to the negotiations, in order to express their own displeasure and concern due to the deterioration of the humanitarian situation and the cessation of the hostilities," de Mistura told reporters.
"They told us, however, their intention to remain in Geneva, in their hotel, and, possibly, at my own suggestion, to pursue technical discussions with myself and my team, particularly on the issues related to Resolution 2254 and the political transition," he added.
However, de Mistura said that he and his team would continue talks with all sides in Geneva, despite the opposition's decision to formally postpone the talks.
"You must know that we plan anyway to continue our discussions and consultations with every side, in the Palais or anywhere else. Proximity talks are very flexible," de Mistura told reporters.
The diplomat added that a meeting would be held Friday to "take stock" of the discussions that have been held so far and decide how to proceed.
"Indeed, if this trend continues — which is worrisome — we will be, obviously, expecting and hoping that the two co-chairs of the ISSG will be convening a special meeting," de Mistura told reporters.
The ceasefire in Syria is generally holding, though fighting in Aleppo is getting worrisome.
"No one can deny that the fighting which is currently taking place in some areas, particularly in Aleppo, is becoming particularly worrisome. Our assessment, however, and this is, I think, an objective assessment, based on our information and an analysis of the whole country…is that the cessation of hostilities is still holding in many areas. But the increase in fighting is, indeed, worrisome," de Mistura told reporters.
"Second point, no one can deny also that the humanitarian access is still going too slowly," he added.