The health ministry admitted in a statement that it did not know what killed the girl and afflicted her nine-year-old brother and a 36-year-old schoolteacher in the same province.
China initially denied the girl died from bird flu, saying she succumbed to pneumonia.
While bird flu has killed more than 60 people in Southeast Asia, China has insisted that it has had no human infections even though there have been several cases involving birds.
"The three cases have been diagnosed as pneumonia of unknown cause but we cannot rule out the possibility of human infection of the H5N1 avian influenza," the ministry said.
"As the cause of the illness was difficult to confirm, our country has invited WHO experts to come to China to jointly carry out further investigation into the cause of illness."
The girl, He Yin, died in late October. The brother and the teacher both recovered.
All three cases were in Xiangtan county in the central province of Hunan, where one of China's four most recent outbreaks of bird flu have been reported.
According to the health ministry, the girl and her brother fell ill last month after they came in close contact with poultry that died. Wantang village, where they lived, was hit by the bird flu virus.
Earlier media reports said the siblings had fallen ill after eating a sick chicken that had died.
The teacher, who lived in a different part of the county, had injured his hand while cutting raw chicken meat before falling ill, the ministry said. All three tested negative in initial tests, it said.
A total of 192 people have been found to have had contact with the three patients and the dead poultry in Wantang village. So far all of them, except for one with acute bronchitis, was in normal health, the ministry said.
The WHO had pressed the government for more data on the dead girl, especially samples taken and tests conducted.
WHO spokesman Roy Wadia told AFP Sunday the global health watchdog saw China's announcement as a "very encouraging" sign.
"If it's confirmed (as bird flu) it will certainly be significant but not surprising," Wadia said.
"It would be a significant and symbolic step...It just means that China is no different from any other country when it comes to bird flu." Wadia said "it would not be surprising" if all three of the Hunan cases turn out to be H5N1 bird flu.
Vice Premier Hui Liangyu meanwhile ordered greater efforts to prevent the spread of the disease.