Four US helicopters lost in Iraq in recent weeks appear to have been downed by ground fire, the US military has admitted publicly for the first time.
The US military relies on helicopters for a variety of missions
A US spokesman in Iraq said that as a result, tactics were being adjusted and mission procedures modified.
Three army helicopters and a private aircraft have come down since 20 January, with the loss of 20 US lives.
The US has lost more than 50 military helicopters in Iraq since May 2003 - about half of them to hostile fire.
Correspondents say the recent incidents have raised new questions about whether insurgents are using more sophisticated weapons, or whether US tactics need changing.
In other developments in Iraq:
- The Iraqi government has said it will announce guidelines for the new security plan for Baghdad, led by Iraqi forces with support from the US, in parliament in the next few days.
- The UN Secretary-General's special representative in Iraq, Ashraf Qazi, has urged Iraqis to show wisdom and courage in finding a way out of the spiralling violence.
Iraqis in Najaf bury the dead from Saturday's Baghdad bombing
- Syrian officials have denied allegations by the Iraqi government that Damascus is not doing enough to stop Sunni militants carrying out attacks in Iraq.
- Residents of Baghdad's al-Sadriya district have been taking the bodies of some of those killed in Saturday's market bombing which claimed at least 130 lives to the holy city of Najaf for burial
- Gunmen killed an ally of Shia cleric Moqtada al-Sadr in the southern city of Basra. Sheikh Khalil al-Maliki was ambushed by three men, police said.
- Fresh bomb attacks in Baghdad on Sunday killed at least 15 people, including four policemen.
An interior ministry official told Associated Press news agency about 1,000 Iraqi civilians, security personnel and militants had died in the past week alone.
Maj-Gen William Caldwell told reporters in Baghdad that although investigations were incomplete, it appeared the helicopter crashes "were all the result of some kind of ground fire".
2 Feb: Apache helicopter comes down near US air base at Taji, two lives lost
28 Jan: Apache downed while supporting Iraqi troops near Najaf, two lives lost
23 Jan: OH-6A helicopter used by private US security firm Blackwater crashes in Baghdad after coming under "heavy gunfire"; four civilians killed
20 Jan: Black Hawk comes down near Baghdad, with loss of 12 lives.
"We don't see this as a focus just on the multinational force," he said.
"There's been an ongoing effort since we've been here to target our helicopters. Based on what we have seen, we're already making adjustments in our tactics and techniques and procedures as to how we employ our helicopters."
He gave no details of the changes.
On Sunday an al-Qaeda-backed group posted a video on a web site which it said showed its fighters bringing down a helicopter in north-west Baghdad.
Friday's crash killed the two crew.
The authenticity of the Islamic State in Iraq group's video could not be confirmed.
The US military depends heavily on helicopters to ferry soldiers and supplies, as well as to stage air strikes against suspected militants and other missions in support of ground forces.