Russian and Syrian war planes will halt airstrikes on Aleppo for eight hours ahead of a planned "humanitarian pause", Russia's defence minister has said.
Sergei Shoigu announced that strikes will be suspended starting from 10am on Tuesday in preparation for humanitarian corridors to be opened for rebels to leave Aleppo.
Russian and Syrian forces will observe the humanitarian pause between 8am and 4pm on Thursday to allow civilians, Syrian rebels and al-Qaida militants safe passage out of the city, and the neighbouring rebel-held Idlib province.
The US welcomed the eight-hour pause on attacks but said it was "too little, too late".
Despite Russia's announcement of a cessation of strikes, ITV News Correspondent Dan Rivers said plenty of artillery and sniper fire could be heard in Aleppo on Tuesday.
Syrian rebels said they rejected any withdrawal of fighters from Aleppo, insisting that they would fight on.
Zakaria Malahifji, the political officer of the Aleppo-based Fastaqim group, said: "The factions completely reject any exit - this is surrender."
Al-Farouk Abu Bakr, an Aleppo commander in the powerful Islamist group Ahrar al-Sham, added: "When we took up arms at the start of the revolution to defend our abandoned people, we promised God that we would notlay them down until the downfall of this criminal regime."
Jens Laerke, the UN's humanitarian spokesman, said the organisation does not have the guarantees it requires to carry out humanitarian operations in eastern Aleppo and evacuated the sick and wounded.
"When the weapons fall silent, we need all weapons to fall silent," he said.
"We need assurances from all parties to the conflict, not just a unilateral announcement that this will happen. We need everybody to give us those assurances before it is immediately useful for us to do anything meaningful."
UN officials have urged combatants to observe weekly 48-hour ceasefires to allow the city's besieged eastern districts to access humanitarian relief.
However, Russian and Syrian forces have escalated their aerial and ground assault on rebel-held areas.
On Monday, opposition activists said airstrikes killed at least 36 people, including several children.