US military officials in Iraq say a roadside bomb has killed two civilians in the centre of Baghdad.

The US military said three other people were injured by the explosion near the heavily defended Green Zone housing the US embassy and government ministries.

Correspondents say it is the biggest attack there for weeks after a series of army operations to restore security.

The military also said three US soldiers were killed in separate attacks on Tuesday and Wednesday.

Two soldiers were killed by a roadside bomb in Diyala province and a third was shot dead in the city of Mosul.

Rush hour

The roadside bomb in the capital targeted a US convoy near the Green Zone, badly damaging an armored Stryker vehicle.

Witnesses say the bomb was set off at the foot of a watchtower during the morning rush hour, when hundreds of Iraqis were waiting to enter the zone.

Another civilian was killed in a car bombing in northern Baghdad.

Reports say Iraqi troops have seized the west Baghdad headquarters of the Sunni scholars' group, the Association of Muslim Scholars, critics of the Shia-led government.

Officials at the Sunni Waqf organization, which reportedly dispatched the troops, say they want to renovate the Um al-Qura mosque, where the group is based.

Separately, FBI agents investigating the Blackwater controversy have reportedly found that at least 14 of 17 killings by Blackwater employees on 16 September broke the rules governing the use of deadly force by private security firms in Iraq.

No evidence supports Blackwater's claim that its employees were fired upon by Iraqi civilians while they were escorting a convoy in west Baghdad, the New York Times reports.

However, the FBI concluded three of the deaths may have been justified under rules that allow lethal force in response to an imminent threat, it says.

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Baghdad attack