Gunmen shot dead three Sunni clerics in the Shia-majority southern province of Basra, Iraqi officials said on Friday.

Raikan Mahdi, the head of the security committee for the province's Al-Zubair district, said the shooting by "unknown gunmen" also wounded two more clerics.

Mahdi said the attack took place as the clerics headed from provincial capital Basra to Al-Zubair after attending a meeting on preparations for celebrating the Prophet Mohammed's birthday.

Interior Minister Mohammed Ghabban ordered an investigation into the attack, the ministry said in a statement.

It blamed the violence on "forces serving the (Islamic State group) project," a reference to a Sunni jihadist organisation that spearheaded a sweeping offensive that overran large parts of the country.

But the Iraqi Islamic Party, one of the main Sunni political parties, said the attack was carried out by "criminal militias," an apparent reference to Shia fighters Baghdad has turned to for support in its battle against IS.

Shia militiamen have played key roles in operations against the jihadists, but have also carried out kidnappings and extrajudicial killings

While pro-government forces have regained some ground from IS, large areas are still outside Baghdad's control.