By Jim Muir
BBC News, Baghdad
The new operation launched by American and Iraqi troops in Diyala province and its capital Baquba, just north of Baghdad, is one of the biggest campaigns undertaken by US troops since the invasion in 2003.
The operation's goal is to destroy "al-Qaeda's influences"
Codenamed Arrowhead Ripper, the operation involves some 10,000 troops - probably around 6,000 US and 4,000 Iraqis - lifted in by helicopters and backed by massive firepower in the shape of tanks, artillery and close air support.
The first focus of the operation seemed to be on the western quarters of Baquba itself, where fighters from al-Qaeda and likeminded radical Sunni groups have become deeply entrenched in recent months, with government control and services withering away.
First reports from the US-led coalition said 22 "anti-Iraqi forces" had been "engaged and killed" in the early hours of the assault, which began in earnest overnight on Tuesday.
An Iraqi military spokesman later said that 11 "terrorists" had been killed and 21 arrested.
There was no official acknowledgement of civilian casualties.
But the head of Baquba's emergency services told the BBC not long after the operation began that at least 12 civilians had been killed by the end of the first day, including three women.
He said that there were certainly more civilian casualties, but ambulances were being prevented by US troops from going in to evacuate them.
A number of houses had been destroyed, and there were fears that civilians might be trapped in the rubble.
People had been told by Iraqi army loudspeakers to keep off the streets and stay indoors.
The goal of the operation, spelled out by the commander of coalition forces in the area, Brig Gen Mick Bednarek, was "to destroy the al-Qaeda influences in this province and eliminate their threat against the people".
A number of suspects were arrested on Tuesday
US military commanders made it clear that this meant killing or capturing Sunni militants rather than allowing them to escape, regroup and return once the heat was off, as has happened elsewhere in the past.
Part of the plan seems to be to bring about a swift restoration of Iraqi government services and activities, which have been virtually suspended in some areas for months, with provincial council budgets largely unspent.
One of the priorities, Gen Bednarek said, would be "getting the Iraqi ministries engaged to provide fundamental goods and services, such as food, fuel, displaced persons support, and education".
The collapse of state services left a vacuum which insurgents were able to exploit in Baquba, just as both Shia and Sunni factions have done - and continue to do - in parts of
Filling that vacuum with state authority will clearly be a crucial part of the campaign to prevent the insurgents from staging a comeback once the focus has moved on.
In Baquba, US forces had also already been pursuing a tactic which has apparently paid off in Anbar province, west of Baghdad.
Insurgent activity in Anbar has dropped off noticeably since some of the local Sunni tribes and groups turned against the al-Qaeda-related radical Islamist factions, some of which are believed to have moved over to Baquba and Diyala province as a result.
A "Diyala Salvation Council" has been established, similar to that set up by Sunni tribal leaders in Anbar, to resist al-Qaeda and support the government.
Some former insurgents from nationalist Sunni groups - such as the 1920 Revolution Brigade - are reported to have been armed by the US military in exchange for co-operation against the outside Islamist militants.
'Surge' in Baghdad
US officials are clearly hoping that a combination of vigorous military action, the revival of government services and bringing on board hitherto alienated sections of local Sunni opinion may succeed in keeping the hardcore Sunni radicals out.
At least 78 people died in Baghdad's bomb blast near a mosque
The Diyala campaign is part of a broader strategy of tackling a whole belt of territory circling Baghdad, into which it is believed some of the activity of both Sunni insurgents and Shia militias has been displaced by the four-month-old security "surge" in the capital, now coming to its peak.
Although US troop levels, at around 160,000, are now as high as at any time since the invasion, American commanders say it will be some time before the effect is felt.
The American ambassador, and the commander of US forces, are to report to Washington in September with a preliminary assessment of how the plan is going, in both military and political terms.
The stated aim of the "surge" is to "buy time" for Iraqi leaders to take steps towards national reconciliation, which have been slow in coming.
US commanders admit the crackdown has yet to achieve many of its objectives.
As the latest bomb attack in Baghdad - the most deadly for over two months - underlined, the "surge" has not managed to prevent bombers from carrying out spectacular outrages.
The number of bodies being found in the streets of the capital, believed to be mainly those of Sunnis abducted, tortured and killed by Shia militias, has also been on the rise again, after dipping in March and April.
Police said that in the past 24 hours, 21 bodies had been found. On some recent days, the number has been 30 or above. Before the surge, it was often above 50.