Up to 45 people have been killed in clashes between Iraqi security forces and followers of a radical cleric in the holy Shia city of Karbala, signalling divisions among Shia factions as a Sunni insurgency rages.

The clashes erupted when police and army personnel tried to arrest Shia cleric Mahmoud al-Sarkhi, an Interior Ministry intelligence officer said.

Mr Sarkhi and his armed followers have clashed in the past with US forces, Iraqi security forces and supporters of Grand Ayatollah Ali al-Sistani, the most revered Shia cleric in Iraq.

Security forces said they went to arrest Sarkhi after his supporters started blocking roads and manning checkpoints around his neighbourhood in the Shia shrine city, home to the tomb of Imam Hussein, which millions of Shia pilgrims flock to annually.

Mr Sarkhi had published a letter on his website earlier this week criticising Mr Sistani's decree for Iraqis to fight alongside the security forces against Sunni militants.

Ms Sistani issued his decree after the Islamic State in Iraq and Syria (ISIS) swept across parts of northern and western Iraq.