Maria Cheng, a WHO spokeswoman, said on Thursday: "According to our Eastern Mediterranean office, it's been dismissed as a case ... they got back to us that it was not avian influenza."
The girl died on Tuesday in a Kurdish area of northern Iraq near the border with Turkey and Iran after contracting a severe lung infection.
Her hometown of Raniya is just north of a reservoir that is a stopover for migratory birds from Turkey, the site of a recent bird flu outbreak.
Cheng said she understood that the girl had died of cardiovascular disease but that had yet to be confirmed.
"For us it's been discarded," she said.
Mohammed Khoshnow, Kurdistan's health minister, said Kurdish officials had begun to burn and bury dead birds, as well as kill any migratory birds they capture.
The girl's family apparently kept chickens in their house and some of those birds had also died.
The investigation into the girl's death followed concerns that the virus could spread from Turkey, where preliminary tests indicated at least 21 people had contracted the deadly H5N1 strain. Four of them - all children - died.
The UN Food and Agriculture Organisation has warned that bird flu might already have spread from Turkey to neighbouring countries, including Iran and Iraq, but there have been no reported cases in Iraq.
The virus has killed 79 people in east Asia and Turkey since 2003.