WHO officials said Saturday that a 10-year-old girl, who died earlier this week along with her brother, was infected with the virus. Test results for the other sibling, an 18-year-old boy, have not yet been confirmed.
On Wednesday, the WHO confirmed the deaths of seven family members from bird flu in Indonesia, calling it the most significant development yet in the spread of the virus.
However, scientists stressed there is no indication of more widespread human-to-human transmission.
Meanwhile, Nigeria's agriculture ministry is confirming a new outbreak of the deadly strain of bird flu in dead chickens in the northern state of Kano.
Government officials declared Kano free of the virus one month ago.
Some information for this report was provided by AP and Reuters.