Among those killed were aid workers and children.
"According to initial reports, four aid workers and at least seven civilians queueing for emergency assistance in eastern Mosul city have been killed by indiscriminate mortar fire," Lise Grande, UN humanitarian coordinator in Iraq, said in a statement.
"Within the last 48 hours, there have been two separate incidents" that also wounded up to 40 people, Grande said.
"People waiting for aid are already vulnerable and need help. They should be protected, not attacked," she said, adding: "The killing of civilians and aid workers violates every humanitarian principle."
The statement came just a day after Human Rights Watch said that the Islamic State group was "indiscriminately" attacking civilians who did not join the militants in retreating.
"Residents said [IS] members told them in person, by radio, and over mosque loudspeakers that those who stayed behind were 'unbelievers' and therefore valid targets along with the Iraqi and coalition forces," HRW said.
According to the rights group, IS have used mortars, gunfire and explosives to target civilians.
Mahmud al-Sorchi, a spokesman for volunteer fighters from Nineveh province, said the aid workers killed in the mortar attack mentioned by the UN belonged to a local organisation called Faz3a.
This report was corroborated by a Facebook page identified as belonging to the aid group, which said that mortar fire and a roadside bomb in Mosul had killed six of its members.
Iraqi forces backed by a US-led coalition launched an operation on October 17 to retake Mosul, the IS group's last major stronghold in Iraq, from the militants.
Over 100,000 people have been displaced since the battle for Mosul began, however Iraqi authorities have advised civilians to stay at home if possible.
This has prevented the numbers of displaced people from reaching the catastrophic proportions predicted by aid organisations, however has left many exposed to the dangers of the conflict.