BAGHDAD, Iraq — Gunmen posing as police killed two American civilians and their Iraqi translator, all employees of the U.S.-led coalition, at a makeshift checkpoint south of Baghdad, the Polish military said Wednesday. In another southern area, four Iraqi policemen died in a shootout with a local militia.

The deaths at the checkpoint came when the gunmen stopped the car Tuesday night outside Hillah, 60 kilometres south of Baghdad, Polish Col. Robert Strzelecki said. The attackers shot dead the passengers and took the vehicle, he said.

Polish troops later intercepted the car, arrested five Iraqis in it and found the bodies inside, said Strzelecki, speaking from the Camp Babylon headquarters of the Polish-led multinational force in Iraq.

In Baghdad, coalition spokesman Dan Senor said the two Americans were civilian coalition staff. He added some details of the attack reported by Polish officials were incorrect, but would not elaborate.

Checkpoints manned by Iraqis or coalition forces are common on Iraq's main roads, and this appeared to be the first time gunmen have posed as police at a roadblock.

Further south, Iraqi police tried Tuesday night to enter a building where a Shiite militia was holding two civilians in the city of Nasiriyah, a coalition spokesman said. In a gunbattle, four Iraqi policemen were killed and two wounded.

The standoff finally ended when Italian security forces stormed the building, rescued the civilians and arrested eight militia members, the spokesman said. One Italian Carabinieri officer was slightly injured.

The militia, known as Citizens' Security Group, acts as a security force for a number of Shiite political parties. Such militias, which in some towns try to enforce a brand of Islamic law, often have tense relations with the U.S.-trained Iraqi police force.

In the northern town of Qaim, near the Syrian border, gunmen killed two police officers and critically wounded a third while the police were having lunch in a restaurant Wednesday, police said.

Meanwhile, Abu Abbas, the Palestinian mastermind of the 1985 hijacking of the Achille Lauro passenger ship that left a wheelchair-bound American tourist dead, died of natural causes while in American custody in Baghdad, U.S. officials in Iraq said Wednesday.

Abbas, 56, who died Monday, was captured by U.S. forces in April, nearly two decades after being convicted in absentia by an Italian court and sentenced to life in prison for the hijacking.

A statement from the U.S.-led coalition did not elaborate on the cause of Abbas' death or say whether he had been under interrogation. There was an attempt to revive him, but it wasn't successful, it said.

Maj.-Gen. Mark Kimmitt said he expected an autopsy would show Abbas died of natural causes and said the State Department was handling the question of what to do with his body.

Abbas' small Palestine Liberation Front commandeered the Italian cruise ship, demanded the release of 50 Palestinian prisoners in Israeli jails and threw an elderly Jewish American tourist, Leon Klinghoffer, overboard after shooting him.

Meanwhile, Iraqi police arrested a prominent member in the northern Iraq-based militant group Ansar al-Islam, an Iraqi Kurd known as Ayoub al-Afghani, in Baghdad late Tuesday and handed him over to coalition forces, a Kurdish security official in Kirkuk said.

In Baqouba, northwest of Baghdad, a bomb went off near the offices of Iraq's largest Shiite party, wounding two people, said Haithem al-Husseini, a party spokesman.

Al-Husseini, of the Supreme Council for the Islamic Revolution in Iraq, or SCIRI, blamed the attack on former Saddam Hussein loyalists and terrorists "trying to spread chaos in the country."