Pursuing his plan for a “freeze” in Aleppo, UN envoy Staffan de Mistura has obtained the agreement of the Assad regime to a fact-finding mission from the Foreign Ministry to Syria’s largest city.

De Mistura and Foreign Minister Walid al-Moallem reached the accord after talks on Saturday in Damascus.

Aleppo has been divided since rebels took part of the city in July 2012. Thousands of civilians have been killed amid the fighting and bombing by Syrian warplanes.

De Mistura said, through his delegation, that he “hopes to set in motion as soon as possible” his plan for a six-week ceasefire to allow humanitarian aid and pave the way for a political resolution.

De Mistura has met President Assad twice in the past month in pursuit of the proposal. The envoy said after the second meeting, in a reversal of the UN’s position, that Assad is essential to any political solution in Syria.

Since June 2012, the UN had sought an agreement for a transitional government in which Assad would step aside.

Opposition groups have insisted that De Mistura’s “freeze” cover all of northwest Syria, including rebel-held areas. The Assad regime has rejected any extension beyond Aleppo.

Military and political opposition forces in Aleppo on Sunday rejected UN envoy Staffan de Mistura’s proposal of a temporary ceasefire in the northern Syrian city.

Rebels also said they will no longer work with the envoy following his recognition of the President’s “essential” position.

Aleppo’s rebel council reiterated on Sunday, “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.”

They added, “Syria and its people are one and indivisible. The blood of our brothers in Daraa [in the south], in Ghouta [near Damascus], in Homs [in the center], and in other Syrian provinces are no less important than our blood in Aleppo.”