"We refuse to meet with Mr Staffan de Mistura if it is not on the basis of a comprehensive solution to Syria's drama through the exit of (President) Bashar al-Assad and his chief of staff, and the prosecution of war criminals," a newly-formed Aleppo revolutionary commission said.
The political and military grouping was set up Saturday at a meeting in the Turkish border town of Kilis attended by exiled coalition chief Khaled Khoja, other opposition figures and Aleppo civil society representatives.
De Mistura's proposal "falls short of an initiative to resolve the humanitarian crisis of our people targeted by the regime's use of chemical weapons and barrel bombs prohibited by the international community", it said.
The Italian-Swedish diplomat, who has made the Aleppo freeze the centrepiece of his mediation efforts since he was named in July as special envoy to Syria, angered the opposition last month by describing Assad as "part of the solution" to the conflict.
Aleppo's opposition forces on Sunday also turned down preferential treatment for their region over other areas of Syria stricken by the four-year conflict.
"Syria and its people are one and indivisible. The blood of our brothers in Daraa (in the south), in Ghouta (near Damascus), in Homs (central) and in other Syrian provinces are no less important than our blood in Aleppo," they said.
De Mistura on Saturday held talks in Damascus to try to finalise a deal to freeze fighting in the battered second city of Aleppo.
He met Foreign Minister Walid Muallem and agreed to send a delegation from his office in the capital to Aleppo on a fact-finding mission, state news agency SANA said, without giving a date.
The envoy "hopes to set in motion as soon as possible his project" to halt fighting in Aleppo for six weeks, said a member of his delegation.
He has met government officials and opposition chiefs in recent weeks to promote his plan for a temporary truce in Aleppo in order to move aid into the northern city, as a starting point to be expanded to other regions.
Once Syria's commercial hub, Aleppo has been devastated by fighting that began in mid-2012, and the city is now split between loyalist forces and rebels.
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About 220,000 people have been killed in Syria since its conflict began in March 2011 with anti-government protests that spiralled into a multi-sided civil war drawing foreign jihadists.
- 19 Christians freed -
In northern Syria, Kurdish forces have recaptured almost 300 of 350 villages around the strategic border town of Kobane which they also seized back from the Islamic State (IS) jihadist group, a monitor said.
The Syrian Observatory for Human Rights also reported that IS had freed 19 Assyrian Christians from a group of 220 kidnapped last week in the northeastern province of Hasakeh.
Osama Edward, who heads the Assyrian Network for Human Rights, said the 19 arrived in the city of Hasakeh on two buses from Shaddadeh, an IS stronghold.
Edward said an IS religious court decided on Saturday to release them in exchange for a sum of money for each family that IS considers as jizya, or tax, paid by non-Muslims.
He was unable to say how much was paid but recalled that in November IS released Assyrians after receiving payments of $1,700 per person.
De Mistura made a surprise visit Sunday to a church near Damascus in a show of solidarity with the country's Christian minority targeted by jihadists.
An AFP photographer said de Mistura travelled to a Greek Catholic church in Jaramana, southeast of the capital, before winding up his mission and leaving for neighbouring Lebanon.
His visit to Syria coincided with a mass of solidarity with the Assyrians kidnapped in the Tal Tamr area of Hasakeh where IS has seized 10 Christian villages, sending almost 5,000 people fleeing to Kurdish- and government-held areas.
In southern Syria, the army and pro-government forces captured several rebel-held hilltops in Tal Adass, southwest of Damascus, after heavy clashes Sunday, the Observatory said.