Syrian rebels have rejected Russia's offer of a withdrawal of fighters from Aleppo after Moscow announced a halt in air raids which it said was designed to allow insurgents to leave and to separate moderate fighters from extremist militants.

Key points: Rebels deem the offer "surrender" and pledge to fight on

Rebels deem the offer "surrender" and pledge to fight on UN says Russian unilateral statement is insufficient

UN says Russian unilateral statement is insufficient Red Cross says it is unclear how much time is needed to get security clearances

"The factions completely reject any exit — this is surrender," said Zakaria Malahifji, the political officer of the Aleppo-based Fastaqim group.

On Monday, Moscow said that Russian and Syrian forces would pause their attacks on Aleppo for eight hours on Thursday to allow rebels to leave while allowing humanitarian aid to enter.

On Tuesday, Russia halted all air strikes, two days ahead of time.

But the United Nations said that Russia's unilateral plan for a humanitarian pause would not ensure any supplies get into besieged eastern Aleppo because Russia and Syria and other groups fighting in the city have not yet given guarantees of safety for aid workers.

"We need assurances from all parties to the conflict, not just a unilateral announcement that this will happen," UN spokesman Jens Laerke said on Tuesday.

"We need everybody to give us those assurances before it is immediately useful for us to do anything meaningful."

The International Committee of the Red Cross also said it was still unclear how much time would be needed to get security guarantees from all sides, including from rebel fighters.

Meanwhile, Al-Farouk Abu Bakr, an Aleppo commander in the powerful Islamist group Ahrar al-Sham, added that the rebels would fight on.

People inspect a damaged site after an air strike in Aleppo. ( Reuters: Abdalrhman Ismail )

"When we took up arms at the start of the revolution to defend our abandoned people we promised God that we would not lay them down until the downfall of this criminal regime," he said, referring to President Bashar al-Assad's Government.

"There are no terrorists in Aleppo," he said.

Rebels in eastern Aleppo have consistently said that insurgent groups linked to Al Qaeda or inspired by it have no real presence in the opposition-held part of the city.

The Russia-Syria bombing campaign has destroyed most health facilities in the besieged city, where 406 people were killed and 1,384 wounded between September 23 and October 8, according to UN data based on reports from medical facilities.

Reuters