A “humanitarian pause” announced by Russia is to formally come into effect in Syria’s city of Aleppo to allow civilians and rebels leave the city.
Moscow earlier said air strikes by Russia and Syria’s government had been halted two days ahead of schedule.
It also extended the pause by three hours, and suggested it could prolong the measure further.
But a powerful jihadist group has refused to leave besieged rebel-held eastern areas of Aleppo.
Last month, Syrian government forces encircled rebel groups and launched an all-out assault backed by Russia.
Some 2,700 people have been killed or injured in the bombardment since then, according to the Syrian Observatory for Human Rights, a UK-based monitoring group.
Western leaders have suggested that Russian and Syrian air strikes on Aleppo could amount to war crimes.
French President Francois Hollande repeated the claim at Wednesday’s late-night talks with Russian President Vladimir Putin and German Chancellor Angela Merkel in Berlin.
Ms Merkel condemned the strikes as “inhumane”.
Russia rejects the accusations, with President Putin saying the accusations are “rhetoric” that do not take into account the realities in Syria.
In other developments on Wednesday:
Syrian President Bashar al-Assad told Swiss broadcaster SRF a photo of a young boy covered in dust and blood in an ambulance that drew worldwide attention was fake
A Russian naval task force – including an aircraft carrier – is heading towards the UK from the Norwegian Sea as it prepares to be deployed off Syria’s coast
Russia’s military says the “humanitarian pause” will formally start at 08:00 local time on Thursday (05:00 GMT).
It says the originally proposed eight-hour pause will now be extended by three hours until 19:00.
Russian Defence Minister Sergei Shoigu said earlier this week the measure would help “guarantee” the safety of six corridors through which civilians and could leave, and sick and injured people could be evacuated.
At the Berlin talks, President Putin suggested he was prepared to extend the pause in air strikes further.
‘Impossible task’
Rebel fighters have been offered a chance to leave with their weapons via two special corridors.
Mr Shoigu also announced that military experts from Russia and countries who support the opposition – including the US, Turkey and Qatar – would hold talks to try to separate “terrorists” from “moderate” rebels.
However, Syria’s political opposition says that is a virtually impossible task, reports the BBC’s James Longman in Beirut.
Jihadist group Jabhat Fateh al-Sham, known as al-Nusra Front until it broke formal ties with al-Qaeda in July, has rejected the Russian offer to leave the city, vowing to fight on.
A statement by the group sent to the BBC pointed to similar initiatives in the central city of Homs in 2014 and the Damascus suburb of Darayya in August, where evacuations took place after years of siege and bombardment by President Bashar al-Assad’s forces.
It was “clear”, the statement said, that Russia wanted to “cleanse” areas of Sunni Muslims – who form the majority in Syria and dominate the opposition to Mr Assad, a member of the Shia Alawite sect – and to “differentiate” between rebel factions in order to weaken them.
“We choose not to give up our people,” the statement said. “We will not betray their blood and we will continue our jihad until we remove the regime and any plan against that is rejected.”
Mainstream rebel factions have also dismissed Russia’s proposal as a gimmick.
The UN, which has designated Jabhat Fateh al-Sham a terrorist organisation, says there are no more than 900 fighters from the group inside Aleppo, out of a maximum of 8,000 rebels in total.
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