"According to initial reports, four aid workers and at least seven civilians queuing 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."
Iraqi forces launched an operation on October 17 to retake Mosul, the country's last city held by the Islamic State jihadist group, and have retaken areas on its eastern side. But the battle - during which more than 100,000 people have been displaced, with many times that number still believed to be inside the city - is far from over.
The UN announcement came a day after Human Rights Watch said that IS was "indiscriminately" attacking civilians who refused to retreat along with the jihadists in Mosul.
"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," the rights group said. The jihadists have targeted civilians with mortars, explosives and gunfire, HRW said.