Faruq Zarzur al-Juburi, a candidate loyal to Vice President Ayad Allawi, was shot dead by gunmen at his home near Mosul, the former IS bastion in northern Iraq, a local official said.

In a statement on the messaging app Telegram, the jihadist group said Juburi was killed because he was "an atheist".

Local official Salah al-Juburi said the gunmen killed the candidate at his home in the town of Qayyarah, 70 kilometres (43 miles) south of Mosul, after storming his house.

IS had threatened to attack Iraqi polling stations and voters during the parliamentary election that will be held on Saturday, in a statement last month 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.

Juburi is a Sunni Muslim while Allawi is a member of Iraq's majority Shiite community.

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.