Russian and Syrian war planes have halted their air strikes on Syria's besieged city of Aleppo in preparation for a temporary pause in the military push, the Russian defence minister said.

Sergei Shoigu said the halt in the strikes should help pave the way for militants to leave the eastern rebel-held parts of the contested city.

Both Russian and Syrian air raids on Aleppo were suspended at 10am local time, Mr Shoigu said.

He described the suspension as a precursor for the opening of humanitarian corridors.

Moscow on Monday announced a "humanitarian pause" between 8am and 4pm on Thursday to allow civilians and militants safe passage out of the city.

At that time, Russian and Syrian militaries will desist from any offensive actions.

Syrian rebels, including al Qaida militants, as well as the wounded and the sick will be allowed to leave for the neighbouring rebel-held province of Idlib.

"The early halting of air strikes is necessary to declare a 'humanitarian pause'," Mr Shoigu said in a televised statement.

"It will ... guarantee a safe exit of civilians through six corridors and prepare for the evacuation of the ill and the wounded from the eastern part of Aleppo."

Russian president Vladimir Putin's spokesman, Dmitry Peskov, said the halt in the air strikes was a goodwill gesture to pave the way for Thursday's pause.

"The Russian military is offering yet another chance, and we hope that our partners will allow us all to take advantage of that," Mr Peskov said.

The United Nations said Russia has communicated plans for two eight-hour ceasefires in rebel-held parts of Aleppo over "consecutive days" this week.

Spokesman Jens Laerke, of UN humanitarian co-ordinator OCHA, said in Geneva the agency needs assurances from all sides that fighting will stop before it can provide humanitarian assistance to the city.

Moscow made no mention of such a pause on Friday.

Mr Laerke said it would be up to Russia to elaborate on its plans and added that the UN was not told in advance of the Russian announcement.

Aleppo, Syria's largest city and once its commercial hub, has been subjected to the most intense aerial bombardment since the start of the Middle East country's conflict in 2011.

In recent months, the Syrian army has pressed its offensive into the rebel-held eastern part of the city.

Air raids have killed hundreds and caused international outrage.

A Russia-US-brokered ceasefire collapsed last month as the Syrian army launched an offensive on eastern Aleppo under the cover of Russian war planes.

Mohammed Abu Rajab, an Aleppo resident, said air strikes on the eastern neighbourhoods stopped early on Tuesday, just after the city had been subjected to another intense round of air raids.

"There were air strikes throughout the night," said Abu Rajab, who works at a local hospital.

But as the air strikes stopped on the city, they continued as usual against nearby rebel-held villages including Anadan and Daret Azzeh. Syrian activists had no immediate word on casualties.

In Moscow, Mr Shoigu added that Russia is "asking the countries wielding influence with the (Syrian) rebels ... to persuade their leaders to end fighting and leave the city".

He said the Syrian troops will pull back to distances allowing unimpeded exit for those carrying weapons via two corridors, including the main artery of Castello Road.

The Russian initiative should also boost talks between military experts from several nations that are set to open in Geneva on Wednesday, he added.

"Their work will be aimed first of all at separating the 'moderate opposition' from the terrorists and its withdrawal from the eastern part of Aleppo," he said.

Moscow has urged Washington to encourage Syria's Western-backed rebels to sever ties with al Qaida militants.

During a meeting over the weekend co-chaired by Russian foreign minister Sergey Lavrov and US secretary of state John Kerry, foreign ministers from Saudi Arabia, Turkey and Qatar said they would work to separate moderate opposition groups in Aleppo from Syria's former al Qaida affiliate once known as the Nusra Front.

Russian and Syrian officials have since embraced a proposal made earlier this month by the UN Syria envoy, Staffan de Mistura, to allow al Qaida-linked militants to leave Aleppo in exchange for a truce and a local administration for the eastern districts.

Rebels there, along with many residents, have rejected the offer.

Russia's announcement did not include any promises of an extended ceasefire or local administration in and around Aleppo.

US state department spokesman Mark Toner, speaking to reporters in Washington on Monday, said the Russian-Syrian pause planned on Thursday was "a bit too little, too late".

Mr Peskov would not say if the strikes would resume after the pause, saying that depends on whether the rebels can be persuaded to cut ties with militants.

Later, a UN spokesman said that despite the planned "humanitarian pause" in the bombing of Aleppo, the UN lacks the time and sufficient safety assurances to begin providing aid to the besieged city.

Stephane Dujarric said that humanitarian workers needed a pause of at least 48 hours in order to send aid convoys into the area.

"The UN and its partners have been and remain ready to proceed with urgent medical evacuation, provide urgent life-saving assistance when all security assurances are provided," he said. "We don't have the security guarantees that we need right now."