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More than 500 people, including dozens of civilians, have been killed since a major Russian-backed regime offensive in Syria's Aleppo province began this month.

The Syrian Observatory for Human Rights, which relies on a network of sources for its figures, said the toll of 506 included 23 children killed in Russian airstrikes on Aleppo city and its surroundings since the operation was launched on February 1.

"At least 143 pro-regime fighters, 274 rebels and foreign jihadists, and 89 civilians were killed from February 1 until Tuesday night," Observatory director Rami Abdel Rahman told AFP.

He said that 14 Iranian troops and at least three fighters from Lebanon's Hizballah movement had died fighting alongside the forces of Syrian President Bashar al-Assad.

Abdel Rahman added that more than 100 foreign fighters, including members of the Nusra Front - Syria's al-Qaeda affiliate - had died since the recent offensive began.

Fighting on Wednesday raged around Tamura, north of Aleppo city, with intense Russian air raids on several nearby villages, the Observatory said.

Pro-regime forces have made a series of gains this month in Aleppo province, severing rebel supply lines and prompting tens of thousands to flee towards the Turkish border.

The UN has warned that 300,000 people in eastern Aleppo city could be cut off from humanitarian aid if government forces encircle the area, a tactic used by the regime to devastating effect against other former rebel bastions.