Staffan de Mistura hopes the plan will establish truces in Aleppo, once Syria's commercial centre and second-most-populous city, and allow aid into rebel districts.
Aleppo has been split roughly in half between opposition groups in the east and government troops in the west, with the remaining civilians subjected to barrel bombing campaigns. The fighting has severely restricted access for humanitarian aid.
De Mistura met on Sunday with Hadi al-Bahra, head of the western-backed National Coalition for Syrian Revolutionary and Opposition Forces, in Istanbul, according to Badr Jamous, a senior member of the opposition body who was also present at the three-hour meeting.
"We are also interested in freezing war but at the moment he has no clear plan... we asked him for a guarantee that (President Bashar) al-Assad won't just direct his force elsewhere so it turns out we are just giving him a rest in Aleppo."
Monzer Akbik, another senior member of the opposition group who was briefed after the meeting, said there was a discussion over whether the plan should apply to all of Aleppo province or just the city.
He said the opposition body did not outline a final position but that any plan needed to be part of a comprehensive scheme involving a full political transition away from Assad's rule.
De Mistura was also meeting opposition fighting groups in the southeastern city of Gaziantep on Monday, according to Akbik, Jamous, other opposition sources and a senior aid worker.
The cease-fire initiative is risky for the disparate opposition groups. The Syrian government has stated its interest, but diplomats and analysts doubt its sincerity. They say Aleppo could face the same fate as the central city of Homs, where government forces have largely regained control.
The presence of al Qaeda offshoot Nusra Front which has fought both alongside and against other opposition groups and led the offensive near the main front line north of Aleppo, has complicated the picture further.
