Civilians trapped in the Syrian city of Raqqa risk being killed by Islamic State snipers or mines if they try to flee but could be used by the militants as human shields if they remain, a leading humanitarian aid organisation said on Tuesday.

The International Rescue Committee outlined the grim choice facing civilians in Raqqa as the U.S.-backed Syrian Democratic Forces (SDF), which includes Arab and Kurdish militias, launched an offensive to capture the city, Islamic State's de facto Syrian capital.

"The IRC has seen a drop in the number of people escaping Raqqa over the past week, which may indicate ISIS intends to use the 200,000 people still trapped in the city as human shields," the IRC's Middle East Director of Public Affairs Thomas Garofalo said in a statement. ISIS is another acronym for Islamic State.

"Those who try to escape once SDF forces advance further into the city risk being killed from mines and ISIS snipers as well as airstrikes," Garofalo said.

"IRC health teams responding north of Raqqa city have treated people injured by mines as they attempted to cross the frontline."

Islamic State captured Raqqa from rebel groups in 2014.

Civilians have increasingly faced food, water, healthcare and electricity shortages in recent months as the SDF forces have advanced on the city.

The IRC is providing emergency cash and health care to those fleeing the anti-Islamic State operation.