At least 33 people were killed Thursday in an air strike on a market in a rebel-held district of the northern Syrian city of Aleppo, activists said, according to the BBC.

The Syrian Observatory for Human Rights reported that the attack happened in the northern Halak district.

The Aleppo Media Center, which put the death toll at more than 40, said two residential buildings were destroyed.

On Wednesday, government aircraft bombed a school in southern Aleppo, killing 18 people, 10 of them children, reported the BBC.

The United Nations Children’s Fund (UNICEF) expressed outrage at what it said was the “latest wave of indiscriminate attacks perpetrated against schools and other civilian targets” across the country.

“These attacks appear to be escalating, in complete disregard of all the calls that have been made to stop this insane cycle of violence, and to avoid similar breaches of international law,” it said.

Rebel-held areas of Aleppo have come under fierce aerial bombardment since mid-December, as government forces try to end a long-standing stalemate in the city.

Many attacks have seen barrel bombs dropped indiscriminately from helicopters on heavily-populated areas, leaving hundreds of civilians dead and driving thousands from their homes.

The Syrian regime has denied that its army is bombing its own citizens, blaming the violence on “terrorist organizations”, the term used by the regime to describe the rebels trying to oust President Bashar Al-Assad.

Rami Abdul Rahman of the Syrian Observatory, told AFP that on Thursday a fighter jet had fired two missiles within a space of a few minutes at the market in the Halak.

Local activists said a government helicopter had dropped three barrel bombs on the area, which had been packed with shoppers at the time.

A video posted online showed an ambulance trying to make its way across rubble and past badly damaged buildings and the burning wreckages of cars, according to the BBC.

As the civil war has raged in Syria, it has been business as usual in Damascus, with Assad having announced his official entrance into the presidential race this week, running for a third seven-year term in the June 3 elections. 

Six other candidates have now announced their intentions to run against Assad, including the first female presidential candidate. Analysts remain pessimistic over the outcome, however, and maintain that the competition is merely a veneer for legitimacy.