The commission recorded 300 civilian deaths from coalition airstrikes in Raqqa province between March 1 and May 31, according to Karen AbuZayd, an investigator for the Commission of Inquiry on Syria.
Human rights and monitoring groups have warned for months of the rising human cost of the coalition's air war in Syria and Iraq as Isis stake out positions in densely populated civilian areas across what remains of the group's self-proclaimed caliphate.
On March 22, the UN commission recorded 200 civilian deaths at an old school building in the village of Mansoura that was sheltering displaced families from across the province.
"These figures have been corroborated by multiple witnesses," AbuZayd said.
The US military said at the time that it was aware of the reports and was opening an investigation.
The activist group Raqqa is Being Slaughtered Silently, which monitors violence in the province, provided the Washington Post with the names of 40 people who it said were killed at the school in Mansoura.
A representative from the group said the bodies of dozens more people without identification cards were believed to have been buried in mass graves.
The UN refugee agency made a public plea this week for better access to the province, where tens of thousands of civilians need assistance amid the fighting.
The US-led coalition includes military personnel from dozens of countries, among them Britain, France and Australia. According to its latest report, at least 484 civilians have been unintentionally killed by coalition airstrikes since June 2014.
But the Britain-based tracking group Airwars puts that figure eight times higher, claiming that more than 3800 people were killed in that period.
Although the nonprofit group previously tracked casualties from both US-led and Russian strikes, it said it now is concentrating resources on claims relating to the US-led coalition to keep up with their pace.
Fatalities from coalition bombing raids now outstrip those caused by Moscow's warplanes, according to Airwars.
The US military said that it has added five full-time members to a team that monitors civilian casualty claims. The team previously consisted of two full-time and two part-time personnel.
Army Colonel Ryan Dillon said that aviation, intelligence and legal experts were added to help boost the team's response time on civilian casualty reports.
At the UN Human Rights Council today, Pinheiro warned that the fight against Isis forces must not be undertaken at the "expense of civilians who unwittingly find themselves living in areas" where the group is present.