Residents in the area say the death told could be upwards of 200.
Some other social media activists put the record of casualties up to 250 people.
"Manbij is full of residents, a quarter of a million residents, used by ISIL (Islamic State, ISIS, IS, and Daesh) as human shields.Yesterday the coalition struck Al Zahuna neighbourhood, where more than 23 people were killed. Then it struck the western gate of the city, killing 6 people."
"Today (Tuesday) the residents of Toka woke to a horrific massacre. The death toll rose to 212 and the numbers are rising," Hasan al-Nifi, a community leader in Mabij, told Al Jazeera.
The Manbij area has seen intense US-led airstrikes in support of Kurdish-led forces who are trying to seize the city from ISIL, so as to block the armed group's access to the nearby Turkish border.
The London Based Syrian Human Rights Observatory on Monday said that 104 civilians had been killed in airstrikes since the Democratic Forces of Syria (DFS - Syrian Democratic Forces - SDF) launched their campaign in the area at the end of May.
There was no immediate comment from the coalition, the Associated Press reported.
The SDF, an alliance of Arab and Kurdish fighters backed by the US, launched an offensive against ISIL to retake the city of Manbij late last month.
12