At least ten civilians, mostly children, were killed in a Russian airstrike Sunday in war-battered Aleppo city in northern Syria.

A Russian warplane dropped cluster bombs on residential areas in the opposition-held Salhiya neighborhood in Aleppo, local civil defense official Najib al-Ansari told Anadolu Agency.

He said ten people, including seven children, were killed in the attack.

"The strike also left 18 people injured," Ansari said.

Sunday's attack comes hours before a truce deal announced by the United States and Russia is expected to come into effect on Monday.

On Saturday, at least 52 people were killed and scores injured in regime airstrikes on opposition-held neighborhoods in Aleppo and the western Idlib province.

Syria has been locked in a vicious civil war since early 2011, when the Bashar al-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-battered country, according to the UN.

The Syrian Center for Policy Research, however, put the death toll from the six-year conflict at more than 470,000 people.