Syrian opposition activists are reporting heavy bombardment of besieged rebel-held neighborhoods in the northern city of Aleppo saying at least six people have been killed.

The Britain-based Syrian Observatory for Human Rights said Syrian government warplanes and artillery struck more than 20 neighborhoods in east Aleppo killing six people.

The Aleppo Media Council, an activist collective, said 15 people, including children, were killed in Saturday's violence in the country's largest city and former commercial center.

Saturday was the fourth day of renewed assaults by Syrian warplanes on eastern Aleppo districts, a rebel-held enclave of 275,000 people. The onslaught began Tuesday, when Syria's ally Russia announced its own offensive on the northern rebel-controlled Idlib province and Homs province in central Syria.