The US military says a major operation targeting suspected insurgents and foreign fighters in northern Iraq could continue for several days.
US military pictures showed troops involved in the operation
They describe it as the biggest airborne operation in the country since the US invasion in 2003, involving more than 50 aircraft and 1,500 troops.
But correspondents are sceptical, saying the operation, near Samarra, is not as huge as has been suggested.
No air power has been used and it is unclear if there have been engagements.
However, the operation involves more helicopters airlifting American and Iraqi troops into the target area than any similar campaign in the three years since Saddam Hussein was toppled.
The troops carrying out the operation are said to have detained about 40 suspects, but 17 of them were later released.
The Iraqi foreign minister said the aim was to stop insurgents from turning the town into a stronghold.
But a senior Sunni Arab politician criticised the operation, which came a day after the new Iraqi parliament met for its inaugural session.
"[The US forces] are surprising us with meaningless acts at the time Iraqis are looking forward to the first session of the parliament, preferring the political solution, not the military one," Saleh al-Mutleq told Reuters news agency.
A bomb attack last month on the al-Askari shrine in Samarra, 100km (60 miles) north of Baghdad, sparked widespread sectarian violence.
The violence continued on Friday, as gunmen in Baghdad opened fire on Shia Muslims on a pilgrimage to the holy city of Karbala, killing three and wounding two. Three more were wounded in a second attack.
Villages sealed off
The BBC's Andrew North in Baghdad says the Pentagon has been keen to publicise this operation, releasing video footage to the media not long after the assault began.
But it is now clear this was nothing like the air strikes of 2003, and the US military has confirmed no bombs were dropped or missiles fired, our correspondent says.
The operation was described as an air assault, a term the US military uses for bringing in troops by helicopter, although many people initially took the phrase to mean aerial bombing.
There has also been little information about any engagements with the insurgents the Americans believe are operating in the area.
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.
Iraqi police sources said several small villages in the largely desert area had been sealed off.
They quoted local residents as saying they had heard loud explosions from just before dawn on this second day of the operation.
They also said they believed a number of people had been killed or injured.
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.
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.
The commander of US forces in the Middle East, Gen John Abizaid, said the operation was linked to insurgent elements but not specifically aimed at high-profile targets like Abu Musab al-Zarqawi, leader of the al-Qaeda in Iraq militant group.
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.
It also comes shortly before the third anniversary of the US-led invasion of Iraq.