The toll is so high because the government was hurling imprecisely aimed, explosive-laden barrels over residential areas, he said.
The Observatory bases its information on a network of activists on the ground.
Other rights groups have given much higher numbers of casualties.
The government has not commented on its use of the so-called barrel bombs, nor on why it began intensely targeting Aleppo. The attack comes weeks before an international conference is expected to bring together Syria's government and opposition groups seeking to overturn the rule of President Bashar Assad.
Analysts say the attack may be an attempt to bully civilians to expel rebels from parts of the city they seized last July.
Also Wednesday, Syria's state news agency said the oil ministry has signed a deal with Russian oil and gas company Soyuzneftegaz to explore in the Mediterranean Sea, in a boost to the war-ravaged country's economic fortunes.
SANA's report did not say where the deal was signed, though it said the exploration will take place off the Syrian coast.
Most of Syria's oil and gas fields in the country's east are under opposition control, and the country's oil exports almost have stopped.
Russia is one of Assad's strongest international backers.
Israel is already developing recent discoveries of massive offshore deposits in the region, and Lebanon has spoken of trying to develop offshore fields.