"According to information verified by the UN Human Rights Office and the UN Assistance Mission in Iraq, at least 307 people were killed... between 17 February and 22 March," the UN rights office said in a statement.
The UN also has "reports" of another 95 people killed in four western Mosul neighbourhoods between March 23 and 26, it said.
US probes Mosul bombing amid worries of rising civilian deaths
The rights office said it was not in a position to provide a breakdown of the deaths caused by IS violence and air strikes by the international anti-jihadist coalition.
Iraqi forces are being supported by the coalition in an effort to dislodge IS from Iraq's second-largest city.
Baghdad and the US defence department are investigating reports that coalition air strikes have killed dozens if not hundreds of people in recent days.
UN rights chief Zeid Ra'ad Al Hussein said he welcomed the probes and that his office did not directly blame the coalition for any specific deaths.
But he called for "an urgent review of tactics to ensure that the impact on civilians is reduced to an absolute minimum".
Iraq probes reports of civilian deaths in Mosul
With IS reportedly using civilians as human shields in buildings around western Mosul, Zeid's spokesman Rupert Colville noted that it was "clearly not easy" to ensure that non-combatants were spared.
"What we are saying is that in these circumstances you have got to be exceptionally careful," Colville told reporters in Geneva.
Referring to the March 17 bombing of a house in the Al-Jadida neighbourhood that killed a large number of civilians, Colville said IS had packed the building with at least 140 people who were used as shields.
Witnesses said IS had also "booby-trapped the house" with improvised explosive devices, according to the UN.
There was also evidence that IS was forcing families to remain in at least 15 frontline western Mosul homes, using the sites to launch attacks on government forces.
Zeid called the use of human shields "an act of monstrous depravity".
More than 200,000 civilians have fled west Mosul in the past month, according to Iraqi authorities.
But some 600,000 remain in IS-held sectors, which include two thirds of the Mosul's Old City, a warren of narrow streets, according to the United Nations.
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