Faruq Zarzur al-Juburi, 45, was shot dead at his home south of the city of Mosul at dawn, a local official told AFP, with the country on edge just days ahead of nationwide elections.

The Islamic State group -- which was forced out of Iraq only five months ago and has threatened the vote -- quickly said it had assassinated Juburi.

But after probing the attack, Iraqi officials rejected the claim and said the candidate's 18-year-old son had confessed to killing him in a quarrel.

"The murder of candidate Faruq Zarzur al-Juburi... had no links to terrorist crimes," judiciary spokesman Abdel Satar Bayraqdar said in a statement.

"It was the result of a family dispute and the son of the victim has admitted to killing his father."

Juburi, a Sunni, was running on Shiite Vice President Iyad Allawi's National Alliance list.

National elections are set to take place in the Shiite majority country on Saturday.

In a statement on the messaging app Telegram, IS said Zarzur al-Juburi was killed because he was "an atheist".

IS threatened in late April to attack polling stations, voters and candidates, in a statement by its spokesman Abu Hassan al-Muhajir.

"Oh Sunnis... we know that the government of Rafida (a pejorative Arabic term for Shiites) is on the verge of what they call elections," he said.

"Our judgement will apply to those who call for them and participate in them... The voting centres and those in them are targets for our swords, so stay away from them and do not walk nearby," he added.

In a video posted on Facebook the day before he died, Juburi called on voters to be wary of outgoing candidates "and those who buy votes," in a message featuring him carrying his six-year-old son Rayan in his arms.

In the video, he promised "a strong government -- one that will take care of poor, families of martyrs, reconstruction and citizens".

"In a few days, God willing, we will be celebrating victory," he said, sitting on the lawn outside his home.

His Facebook page was inundated on Monday with messages deploring his killing.

The May 12 polls are the first to be held since Iraqi Prime Minister Haider al-Abadi declared victory over the jihadists in December.

IS, which had launched a sweeping offensive in 2014 and at one point controlled one third of Iraq, still holds pockets of desert along the border with Syria.

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