Four Iraqi aid workers and at least seven civilians were killed by mortar fire this week during aid distribution in Mosul, the United Nations said, as the campaign to retake the city from Islamic State continued to make slow and punishing progress.

On Thursday, three vehicle-borne improvised explosive devices (VBIEDs) claimed by Islamic State went off in Kokjali, an eastern suburb that the authorities said they had retaken from the jihadists almost two months ago.

At least two civilians were killed and 20 others wounded, including soldiers, according to local police, a health official and a witness. The death toll was expected to rise.

Amaq news agency, which supports Islamic State, said in a statement circulated online that suicide bombers had targeted the army.

A U.N. statement on the two separate mortar attacks this week that killed aid workers and wounded about 40 people said indiscriminate shelling violated international law.

"People waiting for aid are already vulnerable and need help. They should be protected, not attacked," said Lise Grande, U.N. humanitarian coordinator for Iraq.

"All parties to the conflict - all parties - have an obligation to uphold international humanitarian law and ensure that civilians survive and receive the assistance they need."

She did not assign blame for the attacks, but Islamic State militants retreating from the military offensive have repeatedly shelled areas after they are retaken by the army, killing or wounding scores of residents fleeing in the opposite direction.

The U.S.-backed assault on Mosul, the jihadists' last major stronghold in Iraq, was launched by a 100,000-strong alliance of local forces on Oct. 17. It has become the biggest military operation in Iraq since the 2003 U.S.-led invasion that toppled Saddam Hussein.