The policemen were fighting alongside US and Ukrainian occupation troops on Wednesday in al-Suwayrah town in the Polish-controlled zone, a ministry spokesman said in the Polish capital, Warsaw.
After the exchange of fire, 40 fighters were arrested.
Ten policemen were also wounded, but the US and Ukrainian soldiers did not suffer any casualties, Lieutenant-Colonel Krzysztof Laszkiewicz said. Polish troops were not actively involved, he added.
Poland leads an occupation force of 8000 in south-central Iraq, including 2500 Polish troops.
Fighters in Falluja targeted a
In Falluja, four Iraqi policemen were killed on Wednesday when a homemade bomb targeted a joint US and Iraqi convoy, west of the city, a local hospital security officer said.
The explosion went off at 8am (0400 GMT) on a bridge in Habaniya, said Walid Muhammad, who is responsible for hospital security.
Four policemen were killed and the local police chief was wounded in the attack, he said.
Meanwhile, two people were killed, including a 13-year-old child, and seven were wounded when a mortar round exploded in a Baghdad residential neighbourhood on Wednesday, hospital doctors said.
The Baghdad mortar attack killed
The child was killed when the projectile landed on houses in the Rahmaniya district. Three other children were injured, as well as three men and a woman, said a source at the Karama hospital.
A doctor at the nearby al-Karkh hospital said a dead man had also been brought there after the mortar attack.
To the north of Baghdad, a roadside bomb attack on a US patrol killed one soldier and wounded three others on Tuesday, the US military said on Wednesday.
It said the attack was near Balad Ruz, close to the town of Baquba where a car bomb attack killed at least 51 people on Wednesday.