Nearly 50 air strikes by Russian and Syrian regime aircrafts hit rebel-held areas in and around the city of Aleppo over the weekend.
The Syrian Observatory for Human Rights (SOHR) says dozens of barrel bombs were dropped by helicopter on densely populated districts in rebel-held areas.
Media from the Syrian regime claim the air strikes came in retaliation after Al Qaida's Nusra Front escalated its attacks on regime-held areas with mortars.
Russian media say the mortar attacks killed 40 people and injured around 100 in the PYD-controlled Sheikh Maqsoud neighbourhood on Saturday.
The death from this weekend's attacks was 44.
Bebars Mishal, a civil defence official in Aleppo said at least 32 people were killed in the rebel-held parts of the city and 18 bodies were pulled from flattened buildings in the badly hit Qatrji neighbourhood.
"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."
Unidentified jet crashes in Aleppo
SOHR said an unidentified fighter jet crashed on Sunday in Aleppo's countryside in an area where Nusra Front militants were battling regime and Iranian-backed forces.
SOHR said it was unclear whether the plane crashed due to a technical fault or was hit by a missile fired by militants.
— Syria Today (@todayinsyria) June 5, 2016
Aleppo is the second largest city in Syria and has been attacked many times despite a 'cessation of hostilities' agreement.
Analysts say if regime leader Bashar Assad gains full control of Aleppo, it would protect his so-called leadership which will continue to be bolstered by his allies Russia and Iran.
The regime, that killed 400,000 people and forced over 10 million to flee the country, denounced regional governments and were responsible for collapsing a UN-backed political settlement.