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Karbala is the eighth of 18 provinces to be transferred to local control since the US-led invasion in 2003.

The handover took place as at least 27 people died in a suicide bomb attack on a police headquarters in Baquba, north of Baghdad, according to police.

At least 20 people were hurt. Most victims were police recruits.

Correspondents say the attack bears the hallmarks of the al-Qaeda in Iraq militants, who often target recruits.

'Year of security'

Speaking at the handover ceremony in a sports stadium in the city of Karbala, Iraqi Prime Minister Nouri Maliki said the country had been slow to rebuild its security forces.

"Allow me to say that we are late, very late, to reconstruct, to rebuild our forces for reasons that I do not want to mention here," he said.

But he said this year would be the year of Iraqi security while next year the focus would turn more towards reconstruction.

The BBC's Jim Muir, in Baghdad, says that despite the handover the situation in Karbala is far from perfect, with more than 50 people killed in August in clashes between Shia militias and the police.

Police said the bomber had arrived at the scene of the Baquba attack on a bicycle dressed in civilian clothes concealing a suicide belt.

The police headquarters is in a residential area, and it is thought he was able to get through checkpoints because he had been able to pass for a local resident on his way home.

He blew himself up as recruits waited to begin the day's training session.

A witness told Reuters news agency he saw a cyclist slip through a gap in the compound's concrete fence shortly before the explosion.

"I saw many bodies covered in blood . Some were dying, some had arms and legs blown off," said shopkeeper Ali Shahine.

A doctor at the local hospital told the Associated Press news agency that most victims were struck by iron balls packed with explosives to achieve maximum casualties.

Seven of the wounded were in critical condition and unlikely to survive, he added.

Baquba is the capital of Diyala province, where some local tribes have recently joined US and Iraqi forces to fight al-Qaeda in Iraq.

On Sunday, nine tribal leaders from Diyala province were kidnapped while returning from Baghdad.

Baquba's chief of police was killed in a suicide attack last month in a mosque compound during reconciliation talks between Sunni and Shia groups.

But our correspondent says levels of violence have generally been falling since a US troop surge began in February.