Although the leaders said they would consider "all available options" as they headed into their 20-21 October summit, they failed to respond to Moscow's military offensive in Aleppo, Syria's second-largest city now lying in ruins following combined Russia-Syrian government aerial bombing.

Zeid did not name Russian Federation or the Syrian air force, whose jets have attacked the rebel-held districts of Aleppo for weeks, but his reference was clear.

But there was no sign that civilians or rebels were heeding calls to leave, with Damascus and Moscow accusing opposition fighters of preventing evacuations.

United Nations human rights chief Zeid Ra'ad al-Hussein had earlier told the council that Aleppo had become "a slaughterhouse" after weeks of bombardment.

Since then, more than a quarter of a million people have been killed and more than 10 million displaced across the war-torn country, according to United Nations figures.

The pause in Aleppo fighting was announced by Russian Federation to allow for the evacuation of civilians and fighters, as well as the wounded.

Russian Federation announced a halt to its air strikes from Tuesday and the unilateral ceasefire from Thursday. It claimed members of the Ahrar al-Sham group, which is not proscribed by the U.S., had on Thursday publicly executed 14 local self-government officials who had called on people to leave the city's eastern districts.

Deputy US ambassador Ted Allegra said that the recent Russian and Syrian assault on rebel-held areas had killed 400 people, including 100 children.

Syrian state media accused militants of preventing people from leaving the opposition-held side of the city.

But Friday afternoon, a spokesman said the operation had been delayed because of security concerns.

Speaking to reporters in Geneva, Laerke said that the evacuations couldn't begin "because the necessary conditions were not in place to ensure safe, secure and voluntary" movement of people.

The Russian defense ministry said that their forces were observing a truce in Aleppo until 7 P.M. on Saturday to allow evacuation of the wounded and for civilians to leave.

If successful, the plan would then see aid delivered for another two days alongside continuing evacuations.

Syria's representative to the UN Bashar Al Jaafari on Friday accused "terrorist organisations" - primarily the former Al Qaeda affiliate Jabhat Fatah Al Sham and the Islamist Ahrar Al Sham - of attacking the humanitarian corridors and hindering the passage of civilians and fighters out of eastern Aleppo.

A Russian aircraft carrier battle group is now in the North Sea en route from the Baltic to the eastern Mediterranean.

The council was expected to vote later in the day on a resolution that would call for increased monitoring of crimes in Aleppo.

However, very few rebels or civilians appear to have left, sources say.

The Syrian government warned Turkey after deadly strikes against Syrian Kurdish fighters in the north of the country. The sound of gunfire and shelling reverberated in the Syrian capital. He said one fighter was killed.