Two US-led coalition air strikes on towns held by so-called Islamic State in Syria in March killed at least 84 civilians, Human Rights Watch says.
Witnesses were cited as saying that IS militants were present at both sites, along with large numbers of civilians.
The coalition does not believe any civilians died in the Mansoura strike.
It insists that its forces comply with the international law of armed conflict and take all reasonable precautions to reduce the risk of harm to civilians.
At the start of September, the coalition said its strikes in Syria and neighbouring Iraq had unintentionally killed at least 685 civilians since August 2014.
However, an organisation that tracks allegations of civilian deaths, Airwars, says at least 5,343 civilians are likely to have died over the same period.
In a report published on Monday, Human Rights Watch said it had investigated several alleged air strikes during a visit in early July to areas of northern Syria now controlled by a US-backed Syrian Democratic Forces (SDF) alliance.
In two of the deadliest incidents, which took place as SDF fighters sought to encircle the city of Raqqa, coalition aircraft allegedly struck the Badia School in Mansoura on 20 March, and a market and a bakery in Tabqa on 22 March.
Sixteen survivors, witnesses, first responders and medical personnel told HRW that the school in Mansoura was home to a large number of displaced civilians, and that the Tabqa market overwhelmingly served civilians, many of whom were queuing at a bakery at the time of the air strike.
"If coalition forces did not know that there were civilians at these sites, they need to take a long, hard look at the intelligence they are using to verify its targets because it clearly was not good enough," said Ole Solvang, HRW's deputy emergencies director.
HRW quoted the coalition as saying its forces had targeted what they believed to be an IS intelligence headquarters and weapons storage facility in Mansoura. However, the coalition found the allegations of civilian casualties were not credible.
The coalition is still assessing the Tabqa market incident, but HRW's report said a US military spokesperson had acknowledged shortly afterwards that there had been coalition strikes in the vicinity.
HRW's report also criticised the coalition's methodology for assessing civilian casualties, concluding that the information its forces provided suggested that they had not visited the sites in areas under their control or interviewed residents.