The attack took place late Sunday in the village of Khanouka near Ash-Sharqat, one of the last areas retaken by government forces from IS last year, 100 kilometres (62 miles) north of the capital, a police officer said.

"The man, aged 80, had just finished praying (and was leaving the mosque) when the jihadists opened fire at him," the officer said, speaking on condition of anonymity.

"Three other people were wounded in the shooting," he added.

There was no immediate claim of responsibility for the attack.

According to the police officer, IS holdouts are still present in the hills of Khanouka and other mountainous and desert regions of Iraq.

Prime Minister Haider al-Abadi declared "victory" in December in the fight against IS, which seized nearly a third of the country in 2014.

But sleeper cells continue to launch attack from sparsely populated areas.

According to Hisham al-Hashemi, an expert on radical Islamist groups, about 2,000 IS jihadists are still active in Iraq.

The elusive leader of the jihadist group, Abu Bakr al-Baghdadi, called on Muslims to wage "jihad" in a purported new audio recording released on August 22.