The United Nations human rights office says it has received reports of a "significant escalation" of Iraqi civilians being killed by Isis (Islamic State) as they try to flee Mosul.

"Credible reports indicate that more than 231 civilians attempting to flee western Mosul have been killed since May 26, including at least 204 over three days last week alone," the UN human rights office said in a statement.

Iraqi forces have retaken most of Mosul since the offensive began in October, but victory has come at a staggering cost, with heavy casualties among troops and civilians, and hundreds of thousands being forced to flee.

In Syria, meanwhile, government warplanes yesterday struck Isis positions west of Raqqa on the southern bank of the Euphrates river yesterday, state media said.

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The United States-backed Syrian Democratic Forces have launched the battle to take the city back from Isis, which took control of the city in 2014.

With tens of thousands of people uprooted by the fighting, the UN has warned of a dire humanitarian situation in Raqqa, with shortages of food and fuel. The YPG, which is part of the SDF, called for international humanitarian aid.