Announced with media fanfare just hours after Iraq's parliament held a brief first meeting that did nothing to end a political stalemate over forming a government, the US military said 50 aircraft were taking part in the operation north of Baghdad.
The US military released to the media photographs of troop-carrying Black Hawk helicopters lined up in a row for the offensive. There were no pictures of warplanes.
A Pentagon official said it was "predominantly" a helicopter operation that involved UH-60 Black Hawk helicopters and other aircraft and the insertion of ground forces.
"Operation Swarmer" involves more than 1,500 Iraqi and US troops and 200 armoured vehicles targeting insurgents active near Samarra, 100 km north of Baghdad.
Between 600 and 700 of the troops involved were Iraqi government forces and the rest were Americans.
Al-Qaeda rebels
The US fears the power vacuum, three months after elections, is encouraging extremist Sunni rebels loyal to Al-Qaeda in Iraq who are bent on triggering sectarian war which could precipitate Iraq's break-up.
US forces recently said a number of Al-Qaeda insurgents were ensconced in the hilly area, known as Hamreen.
Iraqi Foreign Minister Hoshiyar Zebari said the offensive showed
Iraqi forces, some facing accusations of cooperating with the rebels, are increasingly capable of securing the country.
The US military has launched several major offensives against Sunni Arab insurgents since the 2003 invasion that toppled Saddam Hussein, including one involving several thousand soldiers that captured the former rebel stronghold of Fallujah.
There were also a series of assaults in the rebel heartland in western Iraq's Anbar province which failed to hurt the insurgency and infuriated Iraqis who dug their loved ones out of the rubble after US air strikes.
Security crackdowns were also carried out near Samarra, the the site of a bombing attack last month on a Shi'ite shrine that set off sectarian reprisals and pushed Iraq to the brink of sectarian civil war.
Offensive continues
The offensive was "expected to continue for several days as a thorough search of the objective area is conducted," according to a US military statement
"Initial reports from the objective area indicate that a number of enemy weapons caches have been captured, containing artillery shells, explosives, IED-(bomb) making materials, and military uniforms," said the statement.
The US military has issued frequent statements about the capture of arms, but Iraq is still awash with weapons.
As it has done in the past, the US military made a point of saying both American and Iraqi forces were taking part in the operation in an apparent bid to show that rebuilding of Iraqi forces was making progress.
The US has 130,000 troops in Iraq. Washington has said it will begin withdrawing troops as US-trained Iraqi forces take over security.
But US military officials have said few units were capable of fighting insurgents on their own, let alone protecting people from suicide bombings, shootings, and kidnappings.
Parliament sits
With inter-communal tensions still rife in Iraq the country's first full-term parliament since the ouster of Saddam Hussein in April 2003 has opened.
But continuing deadlock on choosing a prime minister, a president and a parliamentary speaker led to the chamber calling it a day after 20 minutes.
Officially, the session, held deep inside Baghdad's fortified Green Zone protected by walls, razor wire and troops, remains open until such a time as a speaker and his deputies are elected by the new 275-strong assembly.
"It has been decided to leave the session open pending political agreement on the designation of a speaker and his two deputies," said Adnan Pachachi, 83, who presided as eldest member.
Outgoing speaker Hajem al-Hassani said "it is difficult to chose a new speaker and his deputies until a deal is reached on the whole government package."
Outgoing Shiite Prime Minister Ibrahim Jaafari suggested that forming the cabinet might take only another month.
More bodies
The summoning of parliament, dominated by the Shiite United Iraqi Alliance, was meant to provide new momentum to talks between rival Shiite, Sunni and Kurdish factions on forming a coalition at a time of worsening sectarian violence which threatens to trigger civil war.
The latest blood-letting came with authorities announcing the discovery of 25 corpses, men who had been shot dead and dumped in different parts of the capital. At least 80 more were found earlier in the week.
The streets of the capital had remained eerily quiet with traffic barred to keep car bombers at bay, while police and army units multiplied their checkpoints across the city.
The government told civil servants to take a holiday and many other residents bunkered down at home, fearing attacks by insurgents bent on proving the caretaker government cannot guarantee security. Most shops remained closed.