It added that a number of civilians were wounded and the death toll is likely to increase as many sustained serious injuries. 
Since late April, the forces of Syrian President Bashar al-Assad, supported by Russian air power, started a massive campaign against rebels in the provinces of Hama and Idlib, the last major opposition strongholds in the country.
The offensive has so far killed 660 civilians, among them 171 children, and displaced more than 300,000 people, according to the observatory.
Earlier in the day, the New-York based Human Rights Watch (HRW) told the Syrian government to stop the collective punishment of people who were placed "arbitrarily" on a terror list by freezing their assets.
"The government should end collective punishment of families, provide evidence of unlawful activity of the people targeted, and allow them to appeal their listing or unfreeze their assets," the watchdog said.
Decree 63 gives full power to the Finance Ministry to freeze the assets of people pending investigation of their crimes as suspected terrorists under Syria's counterterrorism law of 2012, even where they have not been charged with any crime, HRW said. 
"Syria should stop using the counterterrorism law in arbitrary ways that amount to collective punishment," said Lama Fakih, acting HRW Middle East director.
Syrian refugees who fled eight years ago as a result of war to neighbouring countries like Turkey, Jordan and Lebanon are usually afraid to return because of such decrees and punishment they might face from the government of Syrian President Bashar al-Assad.
According to HRW, people affected by the decree had not been notified and only became aware their assets had been frozen or seized when they attempted to conduct a transaction involving their property.
"As with other legal instruments, Syria is using Decree 63 to authorize abusive and arbitrary practices that rob people of their very livelihoods," Fakih said.
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