So-called Islamic State (IS) shot dead 40 civilians on Tuesday in the northern Iraqi city of Mosul after accusing them of treason, the United Nations says.

Their bodies were then hung from electricity poles in several districts, the office of the UN Human Rights Commissioner said, citing sources.

A man was also reportedly shot dead in public in central Mosul for ignoring an IS ban on using mobile phones.

Iraqi security forces are continuing their push to take Mosul back from IS.

The killings of the civilians appeared to have been carried out on the orders of self-appointed "courts", according to the UN report.

The 40 civilians were accused of "treason and collaboration" and dressed in orange clothes marked in red with the words "traitors and agents of the ISF" (Iraqi Security Forces).

The UN says 20 civilians were also shot dead on Wednesday evening at the Ghabat military base in northern Mosul, supposedly for leaking information.

The UN also expressed concern at IS's deployment of teenagers and young boys. Children are apparently seen in an IS video issued on Wednesday shooting dead four people for spying.

IS also announced on 6 November that it had beheaded seven militants for deserting the battlefield in the Kokjali district of eastern Mosul, the UN says.

Among the sources cited for the UN's information was a man who played dead during a mass killing by IS fighters.

UN High Commissioner for Human Rights Zeid Raad al-Hussein called for the government to "act quickly to restore effective law enforcement in areas retaken from ISIL [IS] to ensure that captured fighters and their perceived supporters are dealt with according to the law".

Large quantities of ammonia and sulphur, for possible use in chemical weapons, is being stockpiled by IS and stored near civilians, the UN said.

Pro-government forces launched an offensive to retake Mosul last month. The city has been occupied by IS since 2014.

The operation, now into its fourth week, involves some 50,000 Iraqi security forces personnel, soldiers, police, Kurdish Peshmerga fighters, Sunni Arab tribesmen and Shia militiamen.

Troops have reportedly been consolidating gains made in the eastern outskirts of Mosul, which they entered nine days ago amid fierce resistance.

There have also been allegations of atrocities committed by government forces.

Amnesty International reported on Thursday that men dressed in Iraqi federal police uniforms tortured and killed up to six residents of villages south of Mosul for having suspected ties to IS.

The federal police deny involvement.