Iraqi and US forces have pushed deep into one of Baghdad's most notorious Sunni bastions, making arrests and seizing weapons as part of a new plan to overrun insurgent strongholds in the capital.
The US military, meanwhile, said Wednesday it was probing reports of another helicopter crash near Baghdad.
A US military official said a crackdown on Sunni insurgents in the northeastern Adhamiyah district of the Iraqi capital began Tuesday in an operation involving some 2,000 US troops and hundreds of Iraqi soldiers.
According to an AFP photographer embedded with the US military more than a dozen people were detained overnight and large numbers of weapons seized.
Major Robie Parke of 3-2 Stryker Brigade Combat Team said the security sweep in the Shaab and Ur areas of Adhamiyah marked the start of the much-vaunted crackdown by US and Iraqi forces against Shiite militias and Sunni insurgents in Baghdad.
"Iraqi and US forces conducted clearing operations in Adhamiyah today, which is the beginning of the new security plan," Parke told AFP late Tuesday.
He said close to 2,000 US troops were involved in Operation Arrowhead Strike Six, along with the 1st Brigade of the 9th Iraqi Army Division.
An aide to Prime Minister Nuri al-Maliki who asked to remain anonymous, however, denied that the security crackdown had begun.
The Baghdad plan, announced by US President George W. Bush along with a decision to deploy another 21,500 US combat troops in Iraq, aims to clear bastions held by insurgents and militia who have been responsible for the deaths of tens of thousands of people in the past year.
Iraq's independent daily Al-Mada said Wednesday that the "massive deployment and troop movements on both banks of Tigris yesterday is a clear signal of the actual launch or that the launching of the new plan was near."
Another newspaper, Al-Zaman, said mixed US and Iraqi forces launched raids Tuesday "in Baghdad after surrounding the areas and arrested dozens of people in a step that indicated the launching of the new plan."
One US officer who asked not to be identified told AFP the operation would set the conditions for a "control and retain" force to move into Shaab and Ur and establish combat outposts in conjunction with Iraqi security forces.
The high-ranking US army official stressed that the Baghdad security plan was a process involving a broad range of troop and logistics movements as well as strategic planning.
"The process has already begun. The Baghdad security plan is a process, it's a long operation, it's not going to be a three day operation or a three week operation," the officer said.
"You have to look at the whole process instead of just thinking in terms of a raid, or an operation in one of the districts, it's much, much larger than that."
More US troops have been arriving in Baghdad for the past three weeks while Iraqi forces have also built up in the capital, establishing reinforced security posts at several points in eastern districts on Monday.
Responding Wednesday to reports that a US helicopter had been shot down, a military spokeswoman said: "This is a serious matter and we are looking into it. As soon as we have anything we will let you know."
The crash, if confirmed, would be the fifth by a US helicopter in Iraq in less than three weeks.
Baghdad's security crackdown comes amid a row between France and the United States over when coalition forces should be withdrawn from Iraq.
French Prime Minister Dominique de Villepin said in an interview on the website of the Financial Times that withdrawing US-led forces according to a set timetable was one possible solution to Iraq's problems.
The White House, however, dismissed the suggestion.
"A timetable for withdrawal before the Iraqi Security Forces have the capacity to protect the Iraqi people would lead to greater violence in Iraq and that is not something that we can let happen," said National security council spokesman Gordon Johndroe.
Iraqi and US forces have pushed deep into one of Baghdad's most notorious Sunni bastions, making arrests and seizing weapons as part of a new plan to overrun insurgent strongholds in the capital.