The US military says some 1,500 US and Iraqi troops are involved
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The US military says a major operation targeting suspected Iraqi insurgents and foreign fighters near Samarra could continue for several days.
The biggest airborne operation in Iraq since the US invasion in 2003, it involves more than 50 aircraft and 1,500 Iraqi and US troops.
They are said to have detained about 40 suspects so far, and seized weapons.
The Iraqi foreign minister said the aim was to stop insurgents from turning the town into a stronghold.
A bomb attack on the al-Askari shrine in Samarra, 100km (60 miles) north of Baghdad, last month sparked widespread sectarian violence.
There are no independent reports of the offensive so far.
The US military said the assault, dubbed Operation Swarmer, was intended to "clear a suspected insurgent operating area" north-east of Samarra.
Helicopters were used to carry mostly Iraqi troops into Salahuddin province, where the Pentagon said at least 41 suspected insurgents had been arrested by the end of the day.
No missiles were fired or bombs dropped by the fixed-wing aircraft providing cover, the US military confirmed. It was unclear whether the suspected insurgents had offered resistance.
Weapons caches
Iraqi interim Foreign Minister Hoshyar Zebari told the BBC that intelligence from Iraqi security services suggested insurgents had gathered in the area to plan terror attacks.
"Whenever the insurgents or the terrorists try to establish themselves, to have a safe haven in any part of the country, you see this kind of reaction by the Iraqi forces, supported by the multi-national forces, to clear these areas," he said.
The offensive is expected to last several days "as a thorough search of the objective area is conducted".
The joint US and Iraqi force said it had captured a number of weapons caches, containing shells, explosives and military uniforms.
US military spokesman Sgt Stan Lavery, in Baghdad, told the BBC: "We are trying to achieve denying the insurgency and the terrorists their weapons, and capturing and killing as many of them as we can."
The commander of US forces in the Middle East, Gen John Abizaid, told reporters in Washington that the operation was linked to "some hard al-Qaeda in Iraq nodes and some hard insurgent nodes that need to be dealt with".
But he dismissed suggestions the raid had been aimed at specific high-profile targets like Abu Musab al-Zarqawi, leader of the al-Qaeda in Iraq militant group.
Show of force
The BBC's Andrew North in Baghdad says the operation - which was carried out in daylight and so did not rely on surprise - may have been more about making a show of strength than crushing specific targets.
It won't end till the coalition leaves and lets Iraqis get on with deciding their own affairs in their own way 
The Pentagon may also have been trying to highlight greater co-operation with the Iraqi forces, who made up most of the troops involved, he adds.
The BBC's Adam Brookes in Washington says a major show of force is being carried out in the hope of breaking a cycle of escalating violence which it is feared could lead to civil war.
The Pentagon is keen to demonstrate that US and Iraqi forces can operate jointly and effectively in response to sectarian attacks, he says.
The military operation is also backing up the political message from the Bush administration in recent weeks that progress is being made in Iraq, he adds.
Asked whether President George Bush had ordered the offensive to bolster falling US support for the Iraq war, White House spokesman Scott McClellan said the decision to attack had been made solely by commanders on the ground.
The operation coincides with the US announcement of a new national security strategy - in which it restates a policy of pre-emptive strikes first issued in 2002 and criticised since the Iraq war - and the first session of Iraq's new parliament.
It also comes shortly before the third anniversary of the US-led invasion of Iraq.