Syrian and Russian aircraft step up bombing of Aleppo city

The group, the Syrian Observatory for Human Rights, also said an unidentified war plane had crashed in countryside south of Aleppo, in an area where Islamist rebel fighters are battling the Syrian army and Iranian-backed forces. It had no information on what caused the crash.

A civil defense worker said at least 32 people were killed in the rebel-held parts of the city during the air strikes, with 18 bodies pulled from flattened buildings in the Qatrji neighborhood, the worst hit.

The monitor said dozens of barrel bombs - oil drums or cylinders packed with explosives and shrapnel- were dropped by helicopter on densely populated districts.

"This week-long campaign of bombing is very intense and day by day it's getting worse ... it is the worst we have seen in a while," said Bebars Mishal, a civil defense official in rebel-held Aleppo.

For their part, rebels hit government-held areas of Aleppo in what Syrian media said was an escalation of mortar attacks on the western districts.

State media said attacks on June 5 on Hamadaniyah, Midan and other neighborhoods by insurgents killed at least 20 people, in the second day of intense shelling of government-held areas. The death toll over the whole weekend was at least 44.

Aleppo, the country's largest city before the war, has been divided for years between rebel and government-held zones.

Full control of Aleppo would be a huge prize for President Bashar al-Assad. Russia's military intervention since September has helped to bolster Assad's government.