Mortar fire killed 11 people including four aid workers as civilians gathered to receive assistance in the battleground Iraqi city of Mosul, the United Nations said on Thursday.

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 part of its eastern side, but these areas are still exposed to deadly artillery attacks, bombs and gunfire.

"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."

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Mahmud al-Sorchi, a spokesman for volunteer fighters from Nineveh province, of which Mosul is the capital, said mortar fire had killed aid workers from a local organisation called Faz3a.

A post on a Facebook page identified as belonging to an aid organisation called Faz3a said that mortar fire and a roadside bomb in Mosul had killed six of its members.

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.

Amnesty International highlighted the impact of the Mosul conflict on children, saying they were exposed to injury or death, in addition to witnessing horrific violence.

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"Children caught in the crossfire of the brutal battle for Mosul have seen things that no one, of any age, should ever see," Amnesty's Donatella Rovera said.

More than 100,000 people have been displaced since the battle for Mosul began more than two months ago, but the Iraqi government has encouraged civilians to stay in their homes if possible.

This keeps the number of people from fleeing from reaching the catastrophic proportions estimated by some aid organisations before the Mosul operation began, but also exposes civilians to significantly more danger than they would face if they moved to camps.

Iraq's elite counter-terrorism service punched into Mosul from the east, but progress has since slowed and the battle is far from over.

Forces that made a long advance toward Mosul on the southern front have yet to enter the city, as have those on the northern side.

The immediate area around western Mosul remains open on the ground, though forces from pro-government paramilitary groups have advanced close to the town of Tal Afar, between Mosul and the Syrian border.