Under the plan, rebel-held eastern Aleppo would remain under opposition control if rebel fighters withdrew.
Foreign Minister Walid al-Muallem, who met the UN envoy to Syria on Sunday, called the idea a violation of "national sovereignty".
Earlier, eight children died in government-held western Aleppo after rebels hit a school, state media say.
In a rebel-held area, a barrel bomb killed a family of six, activists say.
Local medics say the victims in the al-Sakhour district suffocated to death because the bomb was laced with chlorine gas.
