Staffan de Mistura was speaking after a meeting with EU foreign ministers in Luxembourg on Monday where he said he "insisted on the fact that the issue of Aleppo is vital and crucial."
"Any type of discussion that ignores Aleppo would be something that history will judge us for," he added. "Mosul
should not be confusing the priority of Aleppo, in other words, if the world is watching Mosul, Aleppo should not be forgotten."
He described the war-torn city as "iconic" and "a special symbol" and the international community should avoid letting it become like Darayya
or Moadamiyah, referring to areas where the government siege -- considered a starvation tactic by opposition activists -- led to the effective surrender of rebels there.
De Mistura's grave warning came as the skies above Aleppo fell quiet Tuesday morning ahead of a planned, unilateral ceasefire, which saw Russian jets grounded to set the stage for a "humanitarian pause" later this week.
"Today the airstrikes of Russia's Aerospace Forces and Syria's Air Force stop in the Aleppo area from 10 a.m.," a statement posted on the Russian Ministry of Defense's Facebook page
said. "The long-term suspension of airstrikes is necessary to introduce a 'humanitarian pause' on October 20."
Residents of the battered Syrian city told CNN they had heard no bombing early on Tuesday.
The temporary pause in airstrikes will allow civilians to flee the city via "six corridors," according to Russian defense minister Sergei Shoigu. He added that Syrian forces will withdraw and give rebels a chance to leave the beleaguered city via a further two corridors.
"We're calling on the leadership of the countries who have influence on the armed groups in the eastern part of Aleppo with a proposal to convince their leaders to stop military actions and leave the city," the statement added.
"Anyone who is really interested in the early stabilization of the situation in the city of Aleppo need(s) to embark on real political steps and not continue shuffling political papers."
Kremlin spokesman Dmitry Peskov told reporters that the planned truce is a "manifestation" of the Russian military's "goodwill."
"This is an obvious continuation of Russian efforts, on the one hand, to fight terrorists in Syria, and on the other, to unblock the situation in Aleppo," he said Tuesday. "It is exclusively a manifestation of goodwill by the Russian military."
The UN says that Thursday's eight-hour pause is not long enough to effect any aid operations.
Family wiped out
The cessation came hours after airstrikes over the rebel-held east of the city killed 20 members of the same family -- including nine children.
The 20 members of the same family were killed in airstrikes on the neighborhood of Marjah in rebel-held east Aleppo Monday morning, according to activists at the Aleppo Media Center (AMC) and the White Helmets group.
Two six-week old babies -- a boy and a girl -- were among those killed, according to a list of names and ages issued by the Aleppo Media Center (AMC).
In recent days, hundreds of civilians have died
amid the Syrian army's renewed offensive on rebel-controlled parts of the city.
A video shows some of the children killed lying shoulder-to-shoulder on the side of a street, wrapped in white cloth. A man squats and kisses one of the infants on the forehead before carrying the bodies away.
Since Sunday, airstrikes have killed at least 45 people in two neighborhoods in Aleppo, the AMC said.
The death toll continued to climb after Moscow vetoed
a United Nations Security Council resolution Saturday to stop the bombing in Aleppo and allow access for humanitarian aid.
As the strikes have continued, Western powers accused Syrian President Bashar al-Assad and his supporters of war crimes.
Both the United States and United Kingdom have mulled
potential economic sanctions against Syria and Russia due to the Aleppo crisis.