The candidate was "shot dead" by armed men at dawn "after they forced their way into his home," said Salah al-Juburi, a local official in Qayyarah, 70km (43 miles) south of Mosul city.

The official named the slain candidate as Faruq Zarzur al-Juburi, 45, a Sunni from Shiite Vice President Ayad 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 6-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 the attack.

Sunni extremists like IS, and Al-Qaeda before it, have long targeted Shiites in Iraq.

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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