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.
"We recorded the civilians deaths caused by the coalition air strike, altogether 300," commission member Karen Abuzayd told journalists.
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 (20 miles) west of Raqa.
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 Wednesday, 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.
"We note in particular that the intensification of air strikes, which have paved the ground for an SDF advance in Raqa, has resulted not only in staggering loss of civilian life, but has also led to 160,000 civilians fleeing their homes," he added.
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.
'War crimes'
"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.
The UN's Syria commission, set up in 2011 shortly after the civil war began, has repeatedly accused the various sides of war crimes and in some cases crimes against humanity.
On Wednesday, 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.
Pinheiro, who in March warned that the evacuation from Aleppo amounted to a war crime, said Wednesday that "there is no voluntariness nor choice when those who stay often face the risk of being either arbitrarily arrested or forcibly conscripted".
The head of the International Committee of the Red Cross's Middle East division, Robert Mardini, meanwhile told reporters Wednesday 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".
He pointed to a fresh ICRC report detailing the horrors of urban warfare, showing that offensives inside cities kill five times more civilians than battles elsewhere.
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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