Russia will not be able to prolong a moratorium on air strikes against targets in Syria's Aleppo if rebels in the city continue their attacks on the ground, Kremlin spokesman Dmitry Peskov said on Tuesday.

"At the moment the pause is continuing, the exit of the civilian population from eastern Aleppo is being enabled, conditions are being created for humanitarian aid," Peskov told a conference call with reporters.

"But all that is impossible if the terrorists continue to fire on neighbourhoods, humanitarian aid routes, launch attacks, and continue to hide behind a (human) shield. That will not permit the continuation of the humanitarian pause."