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.

The group, which rules swathes of territory in an arc from Aleppo in Syria to near the western edge of Baghdad in Iraq, has since shortened its name to Islamic State and declared a 'Caliphate' to rule over the world's Muslims.

Police and troops reinforced by five helicopter gunships surrounded the house but were prevented from entering by Mr Sarkhi's armed followers, the sources said, adding five police officers and about 40 of Mr Sarkhi's supporters were killed.

The sources said that when security forces managed to break into the house after six hours of clashes, they found Mr Sarkhi had escaped during the battle. Mr Sarkhi's supporters posted on the cleric's website a picture of an Iraqi military Humvee vehicle they said they had destroyed in the battle.

Iraqi PM offers amnesty to some tribes

Iraq's Prime Minister Nouri al-Maliki has offered an amnesty to tribes who fought the government, but excluded those who had "killed and shed blood".

"I announce the offer of the amnesty pardon for all tribes and for all people who were involved in acting against state to return to their sanity, and they are welcome," Mr Maliki said in his weekly televised address.

"We will not exclude anyone except those who killed and shed blood."

Mr Maliki also said that he hoped to overcome the challenges blocking the formation of a new government, a day after the new parliament's first session ended without agreement on top government posts.

"A state of weakness occurred but God willing in the next session (planned for next Tuesday) we will overcome it with cooperation and agreement."

Sunni militants, who have taken over swathes of Iraq, are reported to have told other rebel groups in the region to give up their arms and swear an oath of allegiance to their leader.

The move is part of attempts to consolidate control by the group, ISIS, which is now calling itself the Islamic State.

Mr Maliki has also rejected an assertion by the country's autonomous Kurdish region that its control of disputed territory is here to stay.

"No one has the right to exploit the events that took place to impose a fait accompli, as happened in some of the actions of the Kurdistan region. This is rejected."

He was responding to remarks by regional president Massud Barzani last week that there was no going back on Kurdish rule in the oil-rich northern city of Kirkuk and other towns now defended by Kurdish fighters against Sunni militants.

Kurdish forces moved in to Kirkuk and several other areas when federal forces withdrew in the face of a jihadist-led offensive last month.

Mr Barzani later told the BBC in an interview that Iraq's Kurds will hold an independence referendum within months, telling the broadcaster that the time was right as Iraq was already effectively partitioned.