In one incident, a group of Iraqis claimed that soldiers shot four people dead on Wednesday. Another Iraqi was reported to have been killed on 27 February during a US military operation to capture Iraqi fighters.
The incidents, which involved soldiers from the 25th Infantry Division that arrived in Iraq about a month ago, followed the 18 February killing of an Iraqi girl near Kirkuk.
"I don’t think it’s a case of unseasoned soldiers being twitchy on the trigger," US Army spokeswoman Major Josslyn Aberle said on Thursday.
"These are soldiers who have been specifically trained for the combat mission they are facing in Iraq," she said.
Aberle said the military is, however, investigating the complaint by a group of Iraqis that the soldiers killed four Iraqis early on Wednesday.
Earlier, Aberle said that another US soldier has been placed under investigation in relation to another incident in the Kirkuk area in which an Iraqi man was killed while US troops were trying to detain him.
The soldier, who has not been identified, has been confined to his barracks at the US Army post in Kirkuk.
Meanwhile, a US soldier, who allegedly tossed grenades into an officer's tent in Kuwait last year, could face execution if found guilty of the premeditated murder of two officers, the US military said.
The attack occurred two days after the 20 March invasion at a US military camp housing 101st Airborne Division troops near the Iraq-Kuwait border.
Two officers were killed and 14 others injured when grenades were tossed into a tent as they slept.
Sergeant Hasan Akbar was taken into custody after four grenades were found missing from his tent.
Lieutenant General John Vines, commander of the 18th Airborne Corps, on Wednesday referred charges against Akbar to a military court for trial as a capital case carrying the death penalty.
No date was set for the court martial but it is expected to be held this year.