BEIRUT — Syrian and Russian forces pushed on with their bombardment of Aleppo Wednesday, one day after the two militaries announced a full-scale offensive to retake the contested Syrian city.

The Britain-based Syrian Observatory for Human Rights monitoring group says 20 civilians have been killed in the city's rebel-held eastern districts under the fierce bombardment that resumed Tuesday after a three-week hiatus, while local activists counted at least 50 artillery and air strikes since the morning.

The U.N. estimates that pro-government forces have trapped some 275,000 people in Aleppo's eastern quarters in a strict blockade enforced since August, as ground and air forces meanwhile pound the area's hospitals. The U.N. warned last week that food rations inside Aleppo's rebel-held districts could be depleted this week.

Syrian President Bashar Assad said in an interview published Tuesday with Portugal's state-run RTP television channel that his forces were fighting to liberate civilians from "terrorists," while most of the rebels in Aleppo, numbering around 8,000, according to the U.N., are Syrians who have been fighting to overthrow Assad.

Assad also identified U.S. President-elect Donald Trump as a possible "natural ally," if he turned out to be "genuine" about his commitment to fighting terror in Syria.

Trump said he was ready to work with Assad to fight against the Islamic State group in Syria, in the run-up to last week's American presidential election.

Elsewhere in Syria's north, Turkish-backed Syrian opposition forces are close to taking the town of al-Bab, about 35 kilometers (22 miles) northeast of Aleppo, from the Islamic State group, Turkish President Recep Tayyip Erdogan told reporters on Wednesday.

He said the opposition fighters were about 2 kilometers (1.2 miles) from al-Bab.

"The siege is going according to plan," Erdogan said. "There is a resistance there at the moment but I don't think it will last long."

Ankara sent ground forces into northern Syria in August, vowing to clear the border area of both IS and Syrian Kurdish militias.

The president also predicted U.S.-backed Syrian Kurdish fighters — which Turkey views as terrorists because of their affiliation to Kurdish rebels in Turkey — would soon leave the town of Manbij, in keeping with a U.S. promise to Turkey.

More than 300,000 people have been killed in the 5-1/2 year-long Syrian civil war.