BAGHDAD-Suicide attackers have mounted a bomb and gun attack on a prison convoy in Iraq, killing 51 prisoners and nine police officers, security sources said today. Roadside bombs exploded as the convoy was taking the prisoners from the town of Taji to the capital Baghdad 24km (15 miles) away, justice ministry and medical team said. Gunmen opened fire and a battle ensued with security forces. It is thought the prisoners were being evacuated at the time. The prisoners, who were suspected militants, were put in a convoy after mortar rounds hit military bases in Taji at dawn on Thursday, Iraqi officials told AP, speaking on condition of anonymity. As the convoy travelled through a remote area, roadside bombs went off and the militants opened fire. There were no immediate figures for casualties among the attackers. The attack comes almost a year to the day since gunmen stormed prisons in Taji and Abu Ghraib, also in the Baghdad area, freeing hundreds of inmates. Violent unrest in Iraq has surged again in recent weeks, after Sunni Islamist militants seized cities in the north-west and threatened to sweep south to Baghdad. A suicide bomb killed at least 21 people in Baghdad on Tuesday, going off at a police checkpoint at the entrance to the Kadhimiya district as people travelled to a Shia Muslim shrine in the area.