Two Poles and two Americans working for Blackwater Security
Consulting were killed on Saturday when a convoy they were traveling in was ambushed in Baghdad, said the Polish Foreign Ministry on Sunday.
Blackwater is one of the biggest and best-known private security firms working in Iraq.
Earlier, a car bomb exploded on Sunday outside an Iraqi security force base in Taji, north of Baghdad, killing six Iraqis, said a US military official at the scene.
At least 20 others, including US occupation soldiers, were injured.
Violence has not abated in the run
up to the 'transfer of sovereignty'
A fuel truck first exploded in front of the centre, civil defence sources told Aljazeera. Another car then blew up, causing more casualties than initially announced but it was unclear how many more were hurt.
Anti-occupation fighters have repeatedly targeted Iraqi police and security officers.
On Saturday, a rocket-propelled grenade attack on Iraqis queuing up outside an army recruitment centre in the northern city of Mosul wounded 17 people.
Near the northern city of Kirkuk, an Iraqi policeman and a civilian were killed and nine other officers wounded, five of them seriously, police said.
The bomb went off in Tuz Khurmatu, 75km south of Kirkuk, as a police convoy passed a checkpoint on a road frequently used by US troops travelling between Kirkuk and Tikrit.
South of the capital, anti-occupation fighters attacked a police station south of Baghdad on Saturday, killing at least 10
Armed men disguised as police entered the building in the town of Musayyab and forced policemen into a cell at gunpoint before planting explosives.
When locals tried to free the police, the explosives were detonated, witnesses said.
US toll rise
One US soldier was also killed and another wounded in a mortar attack near the Iraqi capital on Sunday, the US military said.
Both from the 13th Corps Support Command, the attack occured around 08:00 GMT.
The death brings the total number of US soldier who have died in Iraq to 828.