At least seven Iraqi policemen died and hundreds others fell ill after suffering food poisoning yesterday in the town of Numaniya, south of Baghdad.
It was not clear if the poisoning at an Iraqi military base was deliberate. Police sources said they arrested four of the cooks on today.
The policemen became ill only minutes after the meal and at least 1,350 of the 2,000 policemen at the base were hospitalised, Lieutenant-Colonel Hasan Nima said.
Elsewhere gunmen in police uniforms killed the brother of Iraq's Sunni vice president Tareq al-Hashemi in northern Baghdad late last night, police and members of the Sunni Islamic Party said today.
Amer al-Hashemi, a major general in the Iraqi army, was at home in Sulaikh when gunmen stormed into the house and killed him.
"Gunmen in police uniforms and in 10 police cars arrested his bodyguards and then entered his house and killed him," a police source said.
Yesterday US and Iraqi troops killed 30 Shia militiamen in fierce street battles in the southern city of Diwaniya.
People were ordered to stay indoors during the day after explosions and the rattle of machinegun fire shook the city's central districts for more than five hours overnight.
The military said an M1A2 Abrams tank was severely damaged in the clashes that erupted after militants opened fire with machineguns and rocket-propelled grenades on US and Iraqi forces trying to arrest a "high-value" target.
The fighting underlined the chronic insecurity gripping Iraq, where an estimated 100 people die violently every day.
Police reported 28 deaths elsewhere in Iraq, including the discovery in Baghdad of the bullet-riddled body of Colonel Thamir Selman, a senior Interior Ministry official.
The US military said three marines had been killed on Friday in western Iraq, and two soldiers were killed on Saturday, bringing to 29 the number killed in the past eight days. US forces typically suffer two to three deaths a day on average.