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".

US State Department spokesman Mark Toner said Aleppo's residents have been subjected to "near constant bombardment and airstrikes" in an effort to "to starve out and to drive out the opposition and civilians".

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.