The transfer of power from the 101st Airborne Division to the Iraqi army will affect policing across a wide area of northern Iraq.
It comes as violence continues to underscore a security crisis facing the capital Baghdad.
American commanders have sent more soldiers into the capital in a renewed bid to curb the surge of sectarian killings and kidnappings.
A US statement on Tuesday said about 6,000 additional Iraqi troops were being sent to patrol Baghdad, along with 3,500 soldiers of 172nd Striker Brigade Combat Team and 2,000 troops from the US 1st Armoured Division, which has served as the theatre reserve force since November.
Diplomats warn that a civil war could erupt in Iraq if the violence cannot be curbed.
US Blackhawk down
A US Black Hawk helicopter crashed in western Iraq and two of the six troops on board were missing, the US military said today.
"The aircraft has been located. Search and rescue efforts are ongoing for two missing crew members. The remaining four are in stable condition," a Marine Corps spokesman said of yesterday's crash, adding the incident did not appear to be a result of “enemy action”.
Shiite mosque hit
Four Iraqis, including two women and a child, were killed overnight and 20 were injured when a mortar shell hit a Shiite mosque in a town north of Baghdad, police said Wednesday.
The front wall and upper storey of the mosque in the town of Baquba were severely damaged in the blast before midnight Tuesday, as were neighbouring buildings, including the homes of the victims.
Rescuers were trying to free another family of four from the rubble.
Baghdad bombs kill 19
Earlier police said four separate roadside bomb attacks had killed at least 19 people in Baghdad.
US troops made new efforts to try to rid the capital of powerful militias and insurgents.
The deadliest bombing killed at least 10 people and wounded 69 in the al-Shorja market in central Baghdad.
Earlier, two blasts targeting police and another aimed at one of Baghdad's busiest bus stations killed nine people.
Eight people were wounded in those attacks.
Emerging differences
The US campaign to bolster security in Baghdad is likely to hit political minefields.
Prime Minister Nuri al-Maliki's government has vowed to confront militias, but must tread carefully as some of the armed groups have close ties to political parties, including ones in his own ruling Alliance.
The Shi'ite Islamist spoke up against his American allies after US and Iraqi forces fought Shi'ite militiamen in Baghdad during a raid on a suspected death squad on Monday.
"This operation is rejected and it was conducted without the agreement of the government, and it does not match the current national reconciliation environment in the country," Maliki told al-Iraqiya state television on Monday night.
The US ambassador to Iraq, Zalmay Khalilzad, said efforts were under way to end sectarian bloodletting.
"Meetings have taken place between forces of armed militias to reach an agreement to have assigned pledges for ending sectarian attacks on each other," he told reporters at a US base in the northern Iraqi city of Tikrit.
"There is more that needs to be done ... There is an effort to reach a moral compact between religious forces, Sunnis and Shi'ites, to ban sectarian violence."
About 100 people are killed every day in Iraq and this is sapping confidence in the new government.
National Security Adviser Mowaffaq al-Rubaie said he was encouraged by the meetings between Shi'ite and Sunni leaders.
"They have sworn on the Koran they will stop killing each
other," he said.
US and Iraqi forces launched the second phase of Operation Together Forward on Tuesday in a bid to reduce violence in Baghdad.
"We must dramatically reduce the level of violence in Baghdad that is fuelling sectarianism," said Major General James Thurman, commander of US-led forces in the Baghdad area, in a statement.
The first phase of the operation, which began on July 9, killed or captured 411 murderers associated with death squads, said the statement.
In further lawlessness, gunmen stormed a bank in Baghdad's northern Adhamiya district and killed at least five people before walking away with the equivalent of $US4,000 ($A5,255).