A day after Staffan de Mistura's meetings in Gaziantep, Turkey, one rebel chief said he was awaiting a "written proposal from the UN, with all the details, before announcing a decision".
De Mistura's spokeswoman, Juliette Touma, said the envoy had found his six meetings "with representatives of the most important armed and unarmed groups from Aleppo to be constructive".
De Mistura announced his plan for a "freeze" in fighting in late October, following a series of failed international efforts to negotiate an end to Syria's conflict which erupted in March 2011.
He has since said Aleppo is a "good candidate" for the bid.
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Aleppo has been divided between regime control in the west and rebel control in the east since mid-2012.
De Mistura's initiative does not include "all the required mechanisms and guarantees", said Qais Sheikh, head of the Revolutionary Command Council (RCC), a coalition of several dozen moderate and Islamist opposition groups.
The envoy "has built his plan on a series of convictions that are different from ours," said Sheikh, one of the leaders who met with de Mistura.
"So we asked him for a written plan that includes all the details so that we can discuss it in the RCC and with our allies in the opposition," he told AFP.
The Syrian government, for its part, has responded with "constructive interest" to the Aleppo plan, de Mistura said last month after talks in Damascus with President Bashar al-Assad.
De Mistura's deputy, Ramzi Ezzedine Ramzi, is to visit Damascus at the end of this week for further discussions on the plan.