At least 300 civilians have been killed, although the actual number is likely higher according to UN officials.
"Civilians are caught up in the city under the oppressive rule of (IS) while facing extreme danger ... Due to excessive airstrikes," Paulo Sergio Pinheiro, head of the UN Commission of Inquiry, told reporters.
The COI said it had documented that hundreds of civilians had been killed in Raqa province by air strikes carried out by a US-backed coalition supporting the Syrian Democratic Forces since March 1.
She said 200 of those deaths happened on a single day, in the March 21 air strike on the town of Al-Masura, about 30 kilometres west of Raqa.
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The investigators, who have never been granted access to Syria, stressed that the death toll provided was only what they had managed to document, and that the true number was likely higher.
Earlier today, Pinheiro told the UN Human Rights Council that the situation for civilians in Raqa was alarming.
"In areas controlled by extremist factions, we are gravely concerned with the mounting number of civilians who perish during air strikes," he said.
IS seized Raqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate".
Tens of thousands of civilians have fled the city and its surroundings since the push to retake the jihadist stronghold began last November, and new waves of displacement are expected as the battle inside the city progresses.
"The imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where ISIL is present," Pinheiro said, using another acronym for IS.
Today, Pinheiro said deals that have led to evacuations of rebel-held districts and towns in Syria "also raise concerns and in some cases amount to war crimes".
A number of evacuation agreements have been struck for Aleppo and towns and villages around Damascus, as well as in Syria's third city Homs.
The government says the deals are the best way to end the six-year war, but the opposition says this amounts to forced displacement.
The head of the International Committee of the Red Cross's Middle East division, Robert Mardini, meanwhile told reporters today that the organisation's decision to help facilitate the evacuation of civilians was a "tough" call.
"Facilitating the evacuation was a dilemma," he said, stressing though that the ICRC had "acted on behalf of the civilian population that lived in terrible conditions".
More than 320,000 people have been killed since Syria's conflict erupted in March 2011 with demonstrations against President Bashar al-Assad.
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