The Islamic State group has denied food and medicine to hundreds of thousands of people and hidden its fighters among civilians since a US-led coalition began launching air strikes in Iraq and Syria, the UN said.

A UN panel investigating war crimes said Syrians and Iraqis are subjected to an Islamic State "rule of terror" from its calculated use of public brutality and indoctrination to ensure the submission of communities under its control, and that the tactics include repeated violations against children and women.

The conclusions from the Independent International Commission of Inquiry on Syria, a four-member panel of independent experts, are based on more than 300 interviews with people who fled or are living in IS-controlled areas and on collected video and photographic evidence.

"Those that fled consistently described being subjected to acts that terrorise and aim to silence the population," said Brazilian diplomat and scholar Paulo Sergio Pinheiro, who chairs the panel.

He said whatever "services" the group provides to civilians is "always in the framework of this rule of terror", similar to criminal organisations that use such means to control populations.

Commission member Vitit Muntarbhorn said the report is meant to amplify the voices of victims, who describe executions, amputations, public lashings and the use of sexual slavery, child soldiers and widespread indoctrination.

The group has "become synonymous with extreme violence directed against civilians and captured fighters", the report said.

Humanitarian groups have been unable to reach almost 600,000 people in IS-controlled Deir el-Zour and Raqqa provinces, it says, and the group has obstructed the flow of medicine, doctors and nurses into Hassakeh province.

"The group deploys its fighters and materiel in close proximity to civilian areas," the report concludes, adding that since US-led air strikes began civilians living in the northern Syrian town of Manbij have described IS fighters positioning themselves in local homes and farms.

The 47-nation Human Rights Council in Geneva authorised the commission to investigate all alleged violations of international human rights law since March 2011 in Syria and to identify whenever possible those responsible, so that they can be prosecuted.