Two US Marine pilots have been killed after their AH-1W Super Cobra attack helicopter crashed in central Iraq early on Saturday morning, Central Command says.
"Preliminary indications are that the crash was not a result of hostile fire. The cause of the crash is under investigation," the statement said.
Names of those killed were being withheld pending next-of-kin notification.
The dead Marines were the latest American casualties in the war against Iraq that the United States launched to oust Iraqi President Saddam Hussein and his government.
The Pentagon's latest casualty figures say 67 members of US military forces have been killed since the war in Iraq started on March 19, and 154 were wounded, 16 were missing and seven were taken prisoner. It was not immediately clear whether that included the two Marine deaths in the helicopter crash.
Three US soldiers were killed when an F-15E Strike Eagle fighter plane may have accidentally bombed American ground forces on Thursday, the Pentagon said on Friday.
Earlier, US military officials said at least one soldier was killed when the plane may have hit a US artillery position south of Baghdad.
US Central Command is investigating the incident.
The Pentagon listed the three killed as Spc. Donald Oaks Jr., 20, of Erie, Pennsylvania; Sgt. 1st Class Randall Rehn, 36, of Longmont, Colorado; and Sgt. Todd Robbins, 33, of Pentwater, Michigan. All were based at Fort Still in Oklahoma.
The Pentagon also identified three soldiers who died from severe wounds sustained on April 3 when a civilian vehicle blew up at a checkpoint in Iraq in what US officials called a suicide attack. A pregnant Iraqi woman and the driver of the vehicle also died in that incident.
The soldiers who were killed were identified as Staff Sgt. Nino Livaudais, 23, of Utah; Spc. Ryan Long, 21, of Seaford, Delaware; and Capt. Russell Rippetoe, 27, of Colorado. All three were assigned to the 3rd Battalion, 75th Ranger Regiment at Fort Benning, Georgia.
The Defence Department also said an Army Special Operations soldier was killed by enemy fire on Wednesday in northern Iraq, but did not release more details. He was identified as Master Sgt. George Fernandez, 36, of El Paso, Texas. He was based at Fort Bragg, North Carolina.