Gunmen today killed five Shia primary school teachers and a driver in a school in Iskandariya, south of Baghdad, a police spokesman said.

"These men were terrorists in police uniform," the spokesman told Reuters. He said the gunmen arrived at the school in two civilian cars, led the teachers and the school driver to a part of the school where no children were present, and shot them.

Sectarian killings are rife in Iraq, where the Shia and Kurdish-dominated government, backed by US forces, is facing an armed insurgency by Sunni Arabs. But school teachers have rarely been targets for attacks.

Also today, a suicide car bomber killed at least 10 Iraqis in an attack near government buildings in Baghdad, police said. The bomber targeted a police checkpoint guarding Iraq's oil and irrigation ministries and national police academy, police captain Nabil Abdel Qadir said.

Seven policemen and three people died aboard a private bus carrying 24 oil ministry employees and their driver. Thirty-six Iraqis were wounded, 14 of them policemen and 22 of them bus passengers, Mr Qadir said.

Government workers are often searched at the checkpoint before being allowed to walk to their offices about 100 metres away.

The oil minister, Ibrahim Bahr al-Uloum, who rushed to the site, said: "The insurgents are targeting Iraqi government employees and worshippers in mosques. These savage acts won't undermine the forthcoming people's referendum on the new Iraqi constitution."

US forces and the Iraqi government today started to release 1,000 detainees from the notorious Abu Ghraib prison to mark the Muslim holy month of Ramadan, which begins next week, in an attempt to win over moderates in the country.

The first 500 prisoners, who are not thought to have been charged or convicted of any crime, were driven out of the prison on public buses this morning. The rest would be freed later this week, the US military said, stressing that none of those let out had faced allegations of serious or violent crimes.

The prisoner release appeared to be part of the Iraqi government's effort to persuade citizens, especially the Sunni minority, to vote in the October 15 national referendum on Iraq's draft constitution.

Approval of the draft constitution would be an important step in Iraq's democratic transformation. But many Sunni leaders and insurgents are calling for a boycott or a "no" vote in the referendum, saying the draft document would leave Iraq's minority Sunnis with far less power than the country's Kurds and majority Shiites.

If two-thirds of voters in just three of Iraq's 18 provinces vote "no" in the referendum, the constitution would have to be rewritten and Iraq's parliament dissolved and replaced in another election.

The release came at the request of Sunni representatives taking part in constitutional talks. The officials asked the Iraqi government to start releasing thousands of prisoners who have been languishing in the jail for months without charge.

Abu Ghraib prison, built by Saddam's regime in the 1970s on the outskirts of Baghdad, was retained as a major detention centre by the US authorities after the occupation of Iraq in 2003. It gained notoriety after a number of US military personnel were charged with humiliating and assaulting detainees there.