As world powers prepared for last-ditch weekend talks on a ceasefire in Lausanne, Russia-backed Assad's warplanes pounded Aleppo again on Thursday after two days of heavy onslaught that killed more than 70 civilians. Seven children were among the 71 civilians killed in strikes and regime artillery fire in eastern districts of Aleppo on Tuesday and Wednesday, the Syrian Observatory for Human Rights monitoring group said.

Speaking to Anadolu Agency, Beibars Meshaal, a pro-opposition civil defense official based in the city, said Russian jets targeted Aleppo's opposition-held al-Fardos and al-Maadi neighborhoods. "For the last two days, Russian warplanes have been striking opposition-held parts of eastern Aleppo," Meshaal said, adding, "Yesterday, they dropped more than 30 deep-penetration bombs on the city."

On Tuesday, at least 43 civilians were killed - and dozens more injured - by Russian airstrikes on opposition-held parts of the city. Since the Assad forces announced its assault on the city on Sept. 22, more than 370 people have been killed during heavy bombardment on the eastern districts, including 68 children, according to an Observatory toll.

Syria has been locked in a vicious civil war since early 2011, when the Bashar Assad regime cracked down on pro-democracy protests, which erupted as part of the "Arab Spring" uprisings, with unexpected ferocity. Since then, more than a quarter of a million people have been killed and more than 10 million displaced across the war-torn country, according to U.N. figures. The Syrian Center for Policy Research, however, a Beirut-based NGO, puts the death toll from the five-year conflict at more than 470,000.

A new diplomatic push will take place this weekend. U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are expected to be joined in Lausanne on Saturday by counterparts from Turkey, Saudi Arabia and Qatar. Then in London on Sunday, U.S. Secretary of State John Kerry will likely meet up with his European counterparts from Britain, France and Germany.

The two world powers worked closely in recent months and reached a deal in mid-September that was billed as the "best chance" to end Syria's bloodshed. However, the truce fell apart after a week, and peace efforts have since struggled to get off the ground. A French-drafted resolution was shot down by Russia in a stormy U.N. session last weekend. Russian President Vladimir Putin lashed out at Paris for the vetoed proposal, accusing it of deliberately putting forward a motion of "fanning hysteria around Russia." Putin dismissed accusations of possible war crimes as "political rhetoric" and warned Western countries against imposing sanctions on Russia. New Zealand on Wednesday presented another draft resolution demanding an end to air strikes on Aleppo.