A 15-year-old Turkish girl who died on Sunday has tested positive for the deadly H5N1 strain of the bird flu virus, health officials say.
The confirmed cases came from contact with infected birds
Fatma Ozcan died in hospital in the eastern city of Van on Sunday.
The latest test brings the number of confirmed cases in Turkey to 20, all of them after contact with infected birds.
Four of them have died. Altogether more than 70 people, most in East Asia, have now died since the outbreak began in 2003.
The disease reached Turkey only in recent months.
But as it spreads closer to Europe, the UN Food and Agriculture Organization has been advising the EU to impose stricter controls over people coming from areas where the virus has been identified.
Passengers arriving at EU airports from infected areas, including Turkey, should be required to sign a declaration that they are not carrying with them any uncooked chicken or eggs, the FAO says.
One senior FAO official, Juan Lubroth - who has just been visiting Turkey - said that, although bird flu had been positively identified in only 13 out of the country's 81 provinces, there was legitimate suspicion that there had been other unreported outbreaks elsewhere in Turkey.
"We are still in time to be able to prevent the virus from being endemic in Turkey if the Turkish veterinary services have enough resources," he said at a briefing in Rome.
Fatma Ozcan died at 1350 (1150 GMT), authorities in the Van hospital said.
She became ill 10 days ago after slaughtering a sick chicken. Her five-year-old brother, Mohammed, also became infected.
Fatma initially tested negative, but a new round of checks was ordered after a positive test on Mohammed.
A doctor treating him told the BBC he is not on a respirator, and his condition is stable.
The two children are from Dogubeyazit, the remote, rural town where three siblings have already died of bird flu.
A mass cull of poultry is under way in almost one third of all provinces to try to stop the virus spreading any further.
A national awareness campaign has also been launched to warn people of the danger.
Scientists analysing the virus in Turkey say it is a particularly nasty form, but one which has been seen elsewhere.
The analysis of a sample from one Turkish case has shown a genetic change which has been seen in previous human cases in Hong Kong and Vietnam.
However, the scientists have said the alteration does not make the virus more likely to pass between humans.
The UN says it hopes an international donors' summit in Beijing on 17 and 18 January will raise pledges for the $1.5bn (£850m) needed to help countries implement programmes to stop bird flu.