There have been reports of atrocities on both sides of the conflict with Amnesty International claiming Iraqi forces executed villagers south of the city
Islamic State militants have executed 40 civilians in Mosul for "treason and collaboration" according to United Nations reports.
The UN Human Rights Office said the bodies of the victims were hung from electrical poles in the eastern Iraqi city.
The Islamic State group also announced it had beheaded six of its own fighters for deserting the battlefield at Kokjali, near Mosul.
The battle to retake Mosul began in late-October when US backed Iraqi and Kurdish forces moved into the outskirts of the city.
Troops have pushed into the east of the city, but it is expected to take weeks of fighting, if not months, to regain full control.
UN Human Rights spokesperson, Ravina Shamdasani said the militant group has committed a list of atrocities in recent weeks with a mass grave south of the city reported to contain around 100 bodies.
The grave was discovered in the town of Hamam al Alil - around 30km from Mosul - on Monday evening.
Ms Shamdasani said there have been reports the militants have stockpiled large amounts of ammonia and sulphur and placed them in civilian locations in Mosul - with the possible intention of using them as chemical weapons.
She said people with explosive belts, thought to be teenagers or young boys, were being deployed in the alleyways of Old Mosul.
Women who have been abducted are being "distributed" to fighters or told they will be used to accompany Islamic State convoys.
There have been reports of atrocities on both sides of the conflict with Amnesty International warning that “fighters wearing Iraqi Federal Police uniforms tortured and extra-judicially executed residents” in villages south of the city.
The human rights organisation said researchers had gathered evidence up to six people were executed without trial - due to suspicions they had ties to Islamic State.
“Deliberately killing captives and other defenceless individuals is prohibited by international humanitarian law and is a war crime,” said Lynn Maalouf, Deputy Director for Research at Amnesty’s Beirut office.
“It is crucial that the Iraqi authorities carry out prompt, thorough, impartial and independent investigations into these crimes under international law, and bring those responsible to justice.”
The UN fears for the safety of up to 1.5 million civilians living in Mosul, after reports that thousands were being rounded up to be used as human shields.
Nearly 1,800 people were killed in and around the city in October, it said. More than 1,100 of them were civilians.