Three US soldiers and an Iraqi guard were killed today in a mortar attack on National Guard headquarters in Samarra, north of Baghdad.
In Baghdad, police said an explosive device detonated in a vehicle in the southern Dora district, killing one person. They had initially described the blast as a car bomb attack.
The latest violence erupted a day after Iraq's interim government announced a new security law giving itself tougher powers to combat a bloody insurgency wracking the country.
Eighteen US soldiers and three Iraqi guards were also wounded when insurgents fired four mortar rounds at the Iraqi National Guard headquarters in Samarra, severely damaging the building, also used by US troops.
A 1st Infantry Division spokesman said US forces used radar to locate the source of the mortar fire and responded with four 120-mm mortar rounds.
The deaths in Samarra, a mainly Sunni Muslim town, brought to 649 the US combat toll in Iraq since the start of last year's war.
The Iraqi National Guard, renamed by the interim government, is a 40,000-strong paramilitary force set up during the US-led occupation, when it was known as the Iraqi Civil Defence Corps.