American soldiers have been involved in heavy fighting against Saddam Hussein loyalists killing up to 100 Iraqis, according to US military officials.
US says local groups are mounting resistance
They say 27 Iraqis were killed after an American tank was fired on with rocket-propelled grenades in Balad, about 60 kilometres (35 miles) north of Baghdad.
And at least 70 Iraqis were killed during a prolonged assault - begun on Thursday - on what the US calls a "terrorist" training camp 150 km (90 miles) north-west of the capital.
The US forces said they had seized more than 70 anti-aircraft missiles and a similar number of anti-tank grenades in the operation so far.
Code-named Peninsula Strike, the US operation designed to stamp out continuing Iraqi resistance in a belt of country by the Tigris River is the biggest since early May.
The BBC's Jim Muir reports from Baghdad that the resurgence of hostilities seems to stem from a combination of two factors:
the fact that during their initial invasion, coalition forces bypassed large areas which now turn out to contain hostile elements
- fighters loyal to the ousted regime or hostile to the coalition have had time to regroup and find ways of hitting back.
US commanders say they are facing a backlash from small groups organising on a local basis, and not a co-ordinated national movement.
As the operations gathered pace, the London-based Quds Press news agency carried what it said was a handwritten message from the former Iraqi leader.
The message - also carrying Saddam's "personal signature" - pledged "a relentless war" against the coalition forces until they were evicted from Iraq.
More than 40 US troops have been killed since 1 May, when President George W Bush declared the war in Iraq effectively over.
The senior US official running Iraq, Paul Bremer, has blamed the continuing attacks against US troops on organised resistance by Baath Party loyalists.
AMERICANS UNDER FIRE
10 June: One killed in RPG attack in Baghdad
8 June: One shot dead in al-Qaim
7 June: One killed, four injured in attack near Tikrit
27 May: Two dead, nine injured in Falluja
The US military, for its part, said this week's operations were part of "the continued effort to eradicate Baath Party loyalists, paramilitary groups and other subversive elements".
According to US defence officials the operation is a sweep similar to those launched in Afghanistan to root out remnants of the Taleban and al-Qaeda and they say more such operations can be expected.
But while US defence officials see the operations of the last few days as a success story they are also a stark reminder of that the level of resistance is greater than many in the Pentagon had bargained for, says the BBC's Nick Childs.
Fighting has also been reported in the northern town of Mosul with a US soldier gravely wounded, according to US officials.
And 74 suspected "al-Qaeda sympathisers" have been arrested near the northern town of Kirkuk.
On Thursday, two US aircraft came down over Iraq.
A key pipeline from northern Iraqi oilfields to Turkey has meanwhile been set ablaze 225km (140 miles) north of Baghdad, near Baiji - but the cause is unclear.
Local sources said that the fire was caused by bombs planted on the pipeline, which runs near Tikrit, Saddam Hussein's home town and power base.
Turkey says the oil pipeline blast was 'sabotage'
Turkish Foreign Minister Abdullah Gul also said the fire on the pipeline, which runs to the Turkish Mediterranean port of Ceyhan, had been sabotaged.
But US military sources said the fire was the result of a gas leak.
US military and Iraqi engineers are working to repair the vital link.