Vatican City (AFP) - Pope Francis appealed on Wednesday for an immediate ceasefire in Syria, calling for "at least" a truce enabling civilians, especially children, to be evacuated, after Aleppo came under fierce air assault.

"I want to underline and reiterate my closeness to all the victims of the inhumane conflict in Syria," he said at his weekly audience in Saint Peter's Square at the Vatican.

"It is with a sense of urgency that I renew my appeal, begging those responsible, with all my strength, for an immediate ceasefire to be imposed and respected at least long enough to consent the evacuation of civilians, especially children, who are still trapped under the ferocious bombardments," he said.

On Tuesday, regime ally Russia carried out its heaviest air strikes in days on Aleppo, killing 25 civilians including four children according to a monitor, and causing massive damage in several residential areas of the city's rebel-held east.

Syria's army announced a bid last month to retake the city, which has been divided since mid-2012.

The assault began after the collapse of a short-lived truce negotiated by Washington and Moscow, and has seen the besieged east of Aleppo come under fierce aerial assault.

Syrian army said last Wednesday it would reduce its bombardment, after days of attacks that killed hundreds of people and destroyed the largest remaining hospital in the east, but heavy bombing began again on Tuesday.