Pope Francis has called for an immediate ceasefire in Syria, urging at least a truce to be made so that the civilians, especially children, can be evacuated, after Aleppo came under severe air strikes.

"I want to underline and reiterate my closeness to all the victims of the inhumane conflict in Syria," the Pope 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 added.

Russia, which helps Syria in its fight against terrorists, carried out on Tuesday its heaviest air raids in days on Aleppo.

Syrian army announced its plan last month to retake the city, which has been divided since mid-2012. Government forces are in control of the west and the militants' positions are in the eastern part of the city.

The Syrian army, with the help of Russian fighter jets, have attempted to free the trapped people of the city. The army began a major offensive on September 22 to end the militants' control over the eastern parts of Aleppo.

The foreign-backed conflict in Syria started in March 2011 and has claimed the lives of more than 400,000 people, according to a UN report.