The opposition-backed Free Syrian Army has rejected a U.N. proposal to halt fighting in the besieged city of Aleppo. The plan laid out by Staffan de Mistura would allow for the delivery of humanitarian aid and the potential launch of wider peace talks between the rebels and the regime of Bashar al-Assad. Speaking at the United Nations, de Mistura outlined some potential steps.

Staffan de Mistura: “The need for focusing on the real threat of terrorism, as defined by the resolution of the Security Council. Second is to reduce violence, and I’ll come back to that one. Three, through the reduction of violence, try to reach as many people as possible in Syria and outside Syria who have been suffering from this ongoing conflict, and through that, hopefully facilitate it and use it as a block in the direction, a building block, of a political solution.”

De Mistura says the regime of Bashar al-Assad has shown “constructive interest.” But speaking to Al Jazeera, a commander with the Free Syrian Army rejected the U.N. truce offer, saying it will only strengthen Assad. A truce in Aleppo between government forces and rebels could help thwart an advance by the Islamic State, which has already attacked nearby opposition areas.