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By F. Brinley Bruton

The United Nation's special envoy for Syria on Friday warned that time was running out for the population of the besieged and devastated city of Aleppo.

"The clock is ticking," Staffan de Mistura said as he called on Russia to leave the creation of any humanitarian corridors around Aleppo to the United Nations and its partners. On Thursday, a Moscow said its forces and those of the Syrian government would open such passages outside the city and offer a way-out for fighters wanting to surrender.

"That's our job," de Mistura said at a press conference in Geneva.

He also questioned whether the desperate population would even use corridors created by Russia and Syria.

Children peer from a partly destroyed home in Aleppo in February.Alexander Kots / AP

"How do you expect people to walk through a corridor — thousands of them — while there is shelling, bombing, fighting?" he added.

He added that no one should be forced to leave Aleppo, but "indeed, some civilians may want to avail themselves of the possibility afforded by the corridor and by the Russian initiative. When they do, it is crucial that they be given the option of leaving to areas of their own choice."

The Russian and Syrian proposal — like similar ones before it — is largely seen by opposition fighters as a publicity stunt and psychological warfare against the rebels.

Related: 300K Face Death or Starvation in Aleppo, U.S. Doctor Warns

U.S. officials have questioned Russian and Syrian claims that their aim in evacuating civilians from Aleppo was to clear the way for humanitarian assistance to reach the city.

"Why would you evacuate a city that you wanted to send humanitarian aid to?" asked one official. "At first glance, that would appear to be a unilateral effort by Moscow and [Syrian President Bashar] Assad to pre-empt [Secretary of State John] Kerry's demand for ending the siege of Aleppo before starting negotiations on the larger issues. If the proposal isn't dead, it seems to be pretty badly wounded."

Humanitarian groups have warned of a major catastrophe if the siege on the rebel-held parts of Aleppo continues. Some 300,000 residents are trapped in the eastern part of the city that is controlled by rebels, according to the United Nations.

More than a quarter of a million people have died and millions have been displaced since March 2011, when Syria's conflict erupted.

The Associated Press and Reuters contributed.