As many as 100,000 civilians are trapped in Syria's Raqa, as US-backed fighters battle to retake the city from the Islamic State group, the UN rights chief warned Wednesday.

"The intense bombardment of Al-Raqa over the past three weeks has reportedly left civilians terrified and confused about where they can seek refuge," Zeid Ra'ad Al Hussein said in a statement, cautioning that "up to 100,000 civilians are effectively trapped as the air and ground offensive intensifies."