The UK-based watchdog group, which relies on a network of activists on ground, said the death toll is likely to rise as a large number of people were critically wounded.
The incident comes as the UN envoy to Syria, Staffan de Mistura, arrived in the capital Damascus a day earlier for a three-day visit to discuss renewed political talks in Syria between the opposition and the Syrian government.
Meanwhile, the Observatory said another 10 people were killed on Sunday by the U.S.-led strikes against IS positions in the eastern Deir al-Zour province, bordering Iraq.
The Syrian troops have recently intensified its strikes against IS-held areas in northern Syria, apparently taking advantage of the strikes by the U.S.-led anti-terror coalition against the terror group.
Source: KUNA