Tens of thousands of residents of Raqqa have fled the city to safer areas of northern Syria, fearing they could be killed in an air raids to route IS from its stronghold, activists told Adnkronos International (AKI).
"Tens of thousands of people have left," local activist Salem Abu al-Yusr told AKI.
"There is mass flight towards areas of the north of Syria that are relatively safe," he said.
IS has been sending reinforcements to the north of Raqqa since a Kurdish-Arab alliance began an offensive there on Tuesday supported by US-led coalition airstrikes and US forces on the ground.
Coalition warplanes have pounded the area since the assault began, carrying out 150 airstrikes on IS positions, the Syrian Observatory for Human Rights said on Friday.
The fighting and bombardment has killed 31 IS fighters so far, said the UK-based watchdog, which relies for its information on a network of activists and doctors inside Syria.
Coalition leaflets dropped on residents advised them to evacuate Raqqa ahead of the offensive, but IS has tightened restrictions on people leaving the city, amid accusations the jihadist group is using the civilian population as human shields.
According to anti-IS activist group Raqqa is Being Slaughtered Silently, residents were paying smugglers 400 dollars (350 euros) each to try to escape the city, where an estimated 300,000 people are still living.
Neither Arab nor Kurdish forces making up the US-backed Syrian Democratic Forces (SDF) militias are capable of capturing Raqqa, according to al-Yusr. The SDF contain at least 25,000 Syrian Kurds and 5,000-6,000 Syrian Arabs.
"Their means are limited, and they won't get a warm welcome there as the Kurds declared they want the city to form part of the planned Kurdish federation they announced in March," al-Yusr said.
IS occupied Raqqa in 2013 and captured swathes of territory in Syria and Iraq during a lightening offensive in 2014 but areas under its control in the two countries shrunk by about a quarter last year, according to US estimates.