GENEVA (Reuters) – Civilians caught up in the battle for the Syrian city of Raqqa are paying an “unacceptable price” and attacking forces may be contravening international law with their intense air strikes, the top United Nations human rights official said on Thursday.

A U.S.-led coalition is seeking to oust Islamic State from Raqqa, while Syrian government forces, backed by the Russian air force and Iran-backed militias are also advancing on the city.

Some 20,000 civilians are trapped in Raqqa where the jihadist fighters are holding some of them as human shields, the world body says.

U.N. High Commissioner for Human Rights Zeid Ra’ad al-Hussein said that his office had documented 151 civilian deaths in six incidents alone in August, due to air strikes and ground-based attacks.

“Given the extremely high number of reports of civilian casualties this month and the intensity of the air strikes on Raqqa, coupled with ISIL’s use of civilians as human shields, I am deeply concerned that civilians – who should be protected at all times – are paying an unacceptable price and that forces involved in battling ISIL are losing sight of the ultimate goal of this battle,” Zeid said in a statement.

“…the attacking forces may be failing to abide by the international humanitarian law principles of precautions, distinction, and proportionality,” he said.

The U.S.-led coalition has said it conducted nearly 1,100 air strikes on and near Raqqa this month, up from 645 in July, the U.N. statement said. Russia’s air force has reported carrying out 2,518 air strikes across Syria in the first three weeks of August, it added.

“Meanwhile ISIL fighters continue to prevent civilians from fleeing the area, although some manage to leave after paying large amounts of money to smugglers,” Zeid said. We have reports of smugglers also being publicly executed by ISIL.”

U.S.-led warplanes on Wednesday blocked a convoy of Islamic State fighters and their families from reaching territory the group holds in eastern Syria and struck some of their comrades traveling to meet them, a coalition spokesman said.

(Reporting by Tom Miles; writing by Stephanie Nebehay; editing by Pritha Sarkar)