A US military spokesman said two marines were also wounded in the explosion that wrecked a Humvee vehicle escorting a convoy carrying engineering equipment.
Meanwhile, Aljazeera aired a video tape on Monday showing what a purported Iraqi group said was the execution of a US soldier.
While uncertainty shrouded his fate, three Turkish hostages walked free after their release by another group.
The group had previously threatened to behead the Turks on Tuesday unless their government told companies to stop dealing with US forces in Iraq. Ankara had rejected the demand.
"Jama'at al-Tawhid and Jihad announces the release of the Turkish hostages for the sake of Muslims in Turkey and their demonstrations against (US President George Bush," a masked man said on a video tape aired by Aljazeera.
A Turkish government official confirmed the three men had been freed. Two more Turks seized in Iraq three weeks ago have told their families they are well and will return to Turkey within a week, Turkish media reported.
Kidnap groups have also threatened to kill a US marine and a Pakistani. The Pakistani's captors said on Sunday he would be beheaded within three days unless Iraqi prisoners were released.
Kurdish guard killed
Earlier on Tuesday, the bodyguard of a Kurdish official was killed in a roadside bomb blast.
Major Ahmad al-Hamawandi, the head of police in the Kurdish district of Azadi in Kirkuk, suffered minor injuries when the bomb exploded as he was heading to work.
Police stations have been frequent
targets by the resistance
In Mahmudiyah, 35km south of the capital, resistance fighters using assault rifles and rocket propelled grenades attacked a police station, killing one officer and a civilian on Tuesday, police said.
Policemen returned fire in the clash, said Satar al-Ghareri, a policeman.
AP Television news footage showed the front of one police car riddled with bullets. In front of it, a minibus had apparently caught fire and its back window were blown out. A nearby pickup truck was also pockmarked by bullets.
In the early hours of Tuesday, at least four strong explosions shook Baghdad, hours after US-led occupation forces transferred power to the interim Iraqi government, an Aljazeera correspondent reported.
Three strong blasts reverberated through the city about 01:35 (20:45 GMT). The cause of the blasts was unknown but it appeared the explosions occurred on the western side of the Tigris river where the US-controlled Green Zone is located.
Blasts had reverberated across
the US-controlled Green Zone
A fourth, smaller blast occurred a few minutes after the initial volley.
Despite the early handover of power to a transitional Iraqi authority, attacks have continued unabated across the war-ravaged country.
One British occupation soldier was killed on Monday and two others were wounded when their convoy passed over an explosive device in the southern city of Basra.
In Ramadi, one Iraqi was killed and two others hospitalised after clashes erupted between resistance fighters and US occupation forces just hours after the handover was announced.
Also on Monday two Iraqis were killed and another wounded in a bomb attack targeting a US military convoy near the Iraqi city of Baquba, 60km north of Baghdad, the city hospital said.