The UN said Friday security concerns had forced it to delay planned evacuations from Syria's Aleppo, as Russia extended a truce that was largely holding for a second day.
Moscow said it was extending the unilateral "humanitarian pause" in the Syrian government's Russian-backed assault on opposition-held east Aleppo until 1600 GMT on Saturday.
But there was no sign that civilians or rebels were heeding calls to leave, with Damascus and Moscow accusing opposition fighters of preventing evacuations.
In Geneva, the UN rights council called for a special investigation into the violence in Aleppo in a resolution fiercely critical of Syria's government.
East Aleppo, which the rebels captured in 2012, has been under siege by the army since mid-July and has faced devastating bombardment by the government and its ally Russia since the September 22 launch of an offensive to retake the whole city.
Nearly 500 people have been killed, more than a quarter of them children, since the assault began. More than 2,000 civilians have been wounded.
The scale of the casualties has prompted outrage in the West, with Washington saying the bombardment amounted to a possible war crime.
Russia announced a halt to its air strikes from Tuesday and the unilateral ceasefire from Thursday.
The Syrian army says it has opened eight corridors across the front line for the more than 250,000 civilians in rebel-held areas to leave, but so far almost none have taken up the offer.
"There has been no movement in the corridors in the eastern district. For the moment, we haven't seen any movement of residents or fighters," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
Syrian state media accused rebels of preventing people from leaving the city's opposition-held sector.
And Russian Foreign Minister Sergei Lavrov said fighters were using "threats, blackmail and brute force" to prevent civilians and rebels evacuating.
The United Nations had hoped to use the truce to evacuate injured people from Aleppo, and possibly deliver aid.
But on Friday afternoon, a spokesman said the operation had been delayed because of security concerns.
"Medical evacuations of sick and injured could unfortunately not begin this morning as planned because the necessary conditions were not in place," said Jens Laerke, spokesman for the United Nations humanitarian office OCHA.
David Swanson, an OCHA spokesman in Turkey's Gaziantep, told AFP a four-day plan had been drawn up to begin with two days of evacuations to west Aleppo, rebel-held Idlib or Turkey.
If successful, the plan would then see aid delivered for another two days alongside continuing evacuations.
No aid has entered Aleppo since July 7 and food rations will run out by the end of October, UN chief Ban Ki-moon warned on Thursday.
The Syrian opposition National Coalition and rebel Free Syrian Army criticised UN policy in a statement, saying it "plays into the Assad regime's plans to empty Aleppo".
It accused the world body of becoming a "tool in the hands of Russia".
Moscow and Damascus have called on civilians to leave so that their offensive can focus on former Al-Qaeda affiliate the Fateh al-Sham Front and the recapture of east Aleppo.
On Friday afternoon, rebel fire into west Aleppo killed one person and wounded three, Syrian state media said.
Moscow has shown no sign of winding down its intervention, despite criticism of the civilian death toll and allegations that hospitals have been hit.
A Russian aircraft carrier battle group was in the Channel between England and France Friday en route from the Baltic to the eastern Mediterranean.
The UN human rights council, at a special session in Geneva, called in a resolution for "a comprehensive, independent special inquiry into the events in Aleppo", and for those responsible for the most serious violations to be identified.
The resolution demanded that warring parties "in particular the Syrian authorities and their allies", allow unrestricted humanitarian access to desperate civilians and "end immediately all bombardments and military flights over Aleppo city".
The Syrian government, for its part, warned Turkey after a series of deadly strikes against Syrian Kurdish fighters in the north of the country.
In a statement, the army threatened to "down by all available means" Turkish aircraft violating Syrian airspace.
Since late August, Turkey has been carrying out an operation targeting both the Islamic State group and Syrian Kurdish fighters considered "terrorists" by Ankara.