The violence on the edge of the southern city erupted a day before millions of Shia are expected to converge on Karbala for the Shia holy day of Arbain.
The Thursday night clashes between foreign troops and followers of Muqtada al-Sadr, the Shia leader whose followers are fighting US-led occupation troops in several Iraq cities, also wounded 21 Iraqis, said Salah Mahdi, head of Karbala's health department.
The six Iranians were killed on the road between Babel and Karbala as their car approached a Polish checkpoint, a Karbala police spokesman said. Five Iranians were killed in a similar incident earlier this week.
US occupation troops are fighting on two fronts in Iraq against Sunni fighters and al-Sadr followers exactly one year after a US-led invasion toppled Iraqi leader Saddam Hussein.
The US-led occupation administration has warned it cannot guarantee the safety of pilgrims. Bomb attacks during a similar Shia religious event last month killed 171 people.
Meanwhile US-led troops reoccupied the eastern Iraqi town of Kut on Friday, witnesses said, two days after Ukrainian forces withdrew from the city centre after clashes with Shia militiamen.
Shia militia engaged occupation
forces in prolonged resistance
Residents said US soldiers were in control of the centre of the town, and convoys of US armoured vehicles were seen on roads to the area.
Late on Thursday the US forces launched a strike on Kut city, similar in intensity to the fighting during the invasion last March, our correspondent reported.
US Apache helicopters bombed the office of al-Sadr in al-Sharqiya area in the city, but there were no casualties.
As the occupation forces returned to their headquarters in the city early on Friday, they were met with violent resistance which destroyed a Humvee vehicle, the correspondent reported.
Eyewitnesses told Aljazeera that the occupation forces sustained casualties in the counter-attack.
In response, US forces, backed by helicopters, re-invaded the city opening fire randomly, the correspondent said.
Medical sources told Aljazeera that three people were injured late on Thursday. They were treated and immediately taken out of the hospital fearing a US raid on it, the correspondent added.
In Baghdad, US forces evacuated police stations and the town hall in the capital's largest Shia Muslim neighbourhood of al-Sadr City on Thursday night, after five days of clashes with radical militiamen.
US troops abandoned the al-Sadr City central police headquarters as well as the al-Karama, al-Sadr, al-Rafidain, al-Nasr, al-Thawra and al-Tahdid stations.
No US troops were visible at the town hall on the district's main thoroughfare, where clashes have raged since Sunday between US troops and militiamen loyal to Shia radical leader Muqtada al-Sadr.
US forces have also surrounded al-Firdous square in Baghdad, imposing a curfew on it, reported our correspondent.