Before Wednesday's incident, fighters had shot down seven helicopters in Iraq since January 20, killing 28 people, mainly US soldiers.
Five were US military aircraft and two belonged to an American security company.
The incidents have raised questions about whether insurgents are using new tactics, such as studying aircraft flight patterns, or have acquired sophisticated weaponry.
"I saw a ball of fire hitting the aircraft and smoke coming out of the helicopter as it came down"
Mahmoud al-Mashhadani, Iraqi police captain
US commanders have said one reason they believe insurgents had focused on helicopters was to try to undermine a security crackdown in Baghdad that began last week.
Mahmoud al-Mashhadani, an Iraqi police captain said he saw a projectile hitting the helicopter before it came down in a rural area known as the Line, 30km north of Baghdad.
"I saw a ball of fire hitting the aircraft and smoke coming out of the helicopter as it came down," Mashhadani said.
|The Najaf attack killed at least 13 [AP]|
Earlier on Wednesday, a car bomb attacked an Iraqi police checkpoint in the Shia holy city of Najaf, killing at least 13 people - three women, three children and seven police - and wounding dozens more, Iraqi authorities said.
Police and hospital officials said the driver of the car detonated his explosive-packed vehicle at a police checkpoint in the Shia city south of Baghdad on Wednesday morning.
Three more US troops were killed in two separate incidents bringing US military deaths to 3,142 in Iraq since the March 2003 US-led invasion, according to the AFP news agency.