At least 51 prisoners and nine policemen have died in Iraq in an attack on a bus.

The bus was transporting prisoners, many of them held on terrorism charges, from a military base in the town of Taji to Baghdad when it was hit by roadside bombs.

Police said the bomb attack sparked clashes with security forces and left at least 60 dead.

The dead include prisoners and policemen, an official at the interior ministry said.

It was not immediately clear who launched the assault on the security convoy.

The attack took place hours before UN Secretary General Ban Ki-moon was to hold talks with Iraqi Prime Minister Nouri al-Maliki.

They are meeting in an effort to spur international mobilisation against a jihadist insurgency.

The assault came a year almost to the day after an attack on two Iraqi prisons freed about 500 inmates.