US troops hit own jet with Patriot missile.
And a US fighter is feared to have killed at least one Allied soldier in a separate friendly fire incident.
A search and rescue mission was under way after reports the US Navy pilot had ejected from his F/A- 18C Hornet before it was hit.
He was on a mission from the aircraft carrier USS Kitty Hawk when he was attacked by a coalition ground missile battery.
The plane went down near Karbala, where in a separate incident a Black Hawk helicopter came down, killing at least six soldiers and wounding four others.
Iraq claimed it shot down the chopper but the US hinted it was another "blue on blue" incident.
A rescue mission was believed to be under way.
America has lost seven aircraft in the war while the British have seen three losses.
US spokesman Brigadier General Vincent Brooks said both crashes were being investigated.
He added: "We do have some casualties - we don't think as a result of hostile fire."
The US Army soldier was killed in an incident involving an F15E Strike Eagle fighter and ground troops. Several others were injured or missing.
At least five of the 27 British deaths in the conflict were caused by friendly fire.
On March 23, the two-man crew of an RAF GR4 Tornado were killed returning from a mission when it was mistakenly engaged by a Patriot missile battery.