"So far I understand that the Government of Syria has approved access to seven besieged areas," said Vanessa Huguenin, spokeswoman for the United Nations Office for the Coordination of Humanitarian Affairs (OCHA).
The areas involved are Deir ez-Zor, Foah and Kafraya in Idleb, and Madaya, Zabadani, Kafr Batna and Madamiyet Elsham in rural Damascus, she said.
"Humanitarian agencies and partners are preparing convoys for these areas, to depart as soon as possible in the coming days," she added.
De Mistura earlier in the day had announced that an aid convoy would be sent Wednesday to several towns under siege by the regime or the rebels.
"It is clear it is the duty of the government of Syria to want to reach every Syrian person wherever they are and allow the UN to bring humanitarian aid, particularly now after so long time," he told journalists after a meeting with Syrian Foreign Minister Walid Muallem.