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A US helicopter was brought down by ground fire and 23 people killed as Britain and Denmark said they would begin withdrawing their troops.
The downing of a Black Hawk helicopter north of Baghdad marked the eighth crash in just over a month, though no casualties were suffered in the latest incident, a senior US military spokesman said.
In the worst single assault on Wednesday, a suicide car bomber struck in the Shiite holy city of Najaf, detonating his explosives as a police patrol stopped him from entering the old city that is home to the revered Imam Ali Mausoleum.
Najaf's governor, Assaad Abu Gilel, said seven police, three women and three children were killed in the blast, which ripped apart the bomber's Chevrolet Caprice and sprayed deadly shrapnel down a busy street.
A medic said the city's Al-Hakim hospital had treated 34 wounded.
Najaf is an almost entirely Shiite city and security is controlled by local Iraqi units rather than US-led forces. It has been spared the worst of the sectarian war raging elsewhere in central Iraq.
In Baghdad, meanwhile, a US spokesman said the Black Hawk seemed to have been hit by ground fire.
“Initial indications appear that it was brought down by small-arms fire and rocket-propelled grenades,” Lieutenant Colonel Christopher Garver told AFP.
The military had spoken earlier of a “hard landing” north of Baghdad but had added that the crash investigation was ongoing.
“Initial reports are that there were nine people on board. An accompanying helicopter has already landed and picked those personnel up. They're all OK,” Major General William Caldwell had told reporters.
Iraqi police officers said the helicopter came down in fields near the town of Tarmiyah, the scene two days ago of a fierce battle between US troops and insurgents, some 30 kilometres (20 miles) north of the capital.
A statement posted on the Internet earlier in the name of an insurgent group — the Jaish al-Mujahedeen (Army of Mujahedeen) — said its fighters had shot down the helicopter at 10:00 am (0700 GMT).
The statement was posted on a website regularly used by Al-Qaeda but its authenticity could not be independently verified.
Eight US helicopters, including two operated by a private security firm, have been lost in Iraq since January 20. Most were shot down, and a total of 27 people have been killed in the crashes.
Three more US troops have died in gunbattles in Iraq, the military said, bringing to 3,142 the number of fatalities in Iraq since the March 2003 invasion, according to an AFP tally based on Pentagon figures.
In Baghdad, more than 90,000 Iraqi and US troops are carrying out a large-scale security operation to quell a year-long bout of bloodletting between rival Sunni and Shiite factions.
Brigadier General Qassim Atta al-Mussawi, spokesman for the operation, said Iraqi and US forces have killed 42 “terrorists” and arrested 246 others in the week since Prime Minister Nuri al-Maliki officially launched the plan.
Caldwell told media: “There has been a significant reduction in sectarian incidents and in extra-judicial killings in Baghdad because the Iraqi people have chosen restraint rather than retribution.”
Despite the plan, however, violence continued in the capital.
A tanker truck carrying chlorine exploded in the west of the city, killing two people, wounding seven and leaving 35 others sick from the effects of the toxic gas, security and medical sources said.
At least eight other people died and 35 were wounded in other attacks in Baghdad and the northern flashpoint city of Kirkuk, a hub of Iraq's oil industry that is disputed by Kurds and Arabs.
In London, Prime Minister Tony Blair announced that British troop numbers in southern Iraq would be reduced from 7,100 to 5,500.
And Danish Prime Minister Anders Fogh Rasmussen said in Copenhagen that Denmark will withdraw all of its 460 troops stationed in Iraq in August.
