The Polish PAP news agency reported the nine soldiers were killed while attempting to detonate a weapons cache in Wasit province on Sunday.
It quoted Gerenal Andrzej Ekiert, commander of the Polish-led multinational division in south-central Iraq, as saying that a further seven Ukrainians and four Kazakhs were wounded in the incident.
In a related incident, South Korean foreign ministry officials were investigating reports that one or more of their citizens have been kidnapped in Iraq.
Separately, in the city of Samarra, 100km north of Baghdad, an unidentified group of armed men shot and killed on Sunday the acting chief of police, Muhammad al-Badri, as he travelled in his car.
Earlier, Iraqi interior ministry sources said two Iraqi policemen and two civilians were killed by US soldiers who opened fire after their convoy was targeted by a roadside bomb.
US troops started shooting after
coming under attack in Yusufiya
The convoy came under attack near Yusufiya late on Saturday.
The spokesman, Adnan Abd al-Rahman, said on Sunday a fifth Iraqi died from a heart attack during the incident.
A US military spokesman had no immediate information on the incident but said it was under investigation.
The US military said on Sunday it was investigating a separate incident in which one of its warplanes bombed the wrong house in northern Iraq, killing as many as 14 civilians and fuelling anti-American anger three weeks before elections.