UN demands answers on 48-hour Aleppo ceasefire by Sunday
Staffan de Mistura has led global calls for the pause that the UN and aid groups like the Red Cross say is desperately needed by civilians trapped in the midst of brutal fighting between regime and opposition forces.
Russia, which backs Syrian President Bashar al-Assad's forces, has endorsed the plan.
De Mistura said in a statement Saturday that he "regrets" some opposition camps have expressed reluctance to agree to the plan, without detailing the nature of their concerns.
"The special envoy calls for all concerned to exert every effort so that, by this Sunday, 28 August 2016, we know where we stand," de Mistura's office said in a statement.
He added that the initial delivery of life-saving aid must be carried out through the strategically crucial Castello Road, which the regime took control of in July, cutting off the last supply route to rebel-held Aleppo.
The UN has "pre-positioned" aid that is ready to go to the city, it said.
The first delivery would benefit 80,000 people in the rebel-held east as well as people in the government controlled west, the statement said.
"The UN is ready to move," it added. "People are suffering and need assistance. Time is of the essence. All must put the civilian population of Aleppo first and exert their influence now."
According to de Mistura, Russia "has engaged" its ally Assad on the plan.
The brutal fight for Aleppo, Syria's second city, intensified two months ago.
After a nearly three-week siege by regime troops, rebels early this month linked up with opposition-held neighbourhoods via a new road from the city's south, in a major blow to forces loyal to Assad.
But fighting has continued near the new supply line, which recently has been bombarded almost daily, affecting supplies coming into the city's opposition-controlled neighbourhoods.
Around 250,000 people live in the city's eastern districts, while another 1.2 million live in its western neighbourhoods.
More than 290,000 people have been killed in Syria since the conflict began more than five years ago.