Indonesia has also reported a new death amid dire warnings from the World Health Organisation (WHO) over a growing threat of a possible influenza pandemic.
The Turkish health ministry said new tests showed that an 11-year-old girl who died last week at a hospital in the eastern Turkish city of Van had succumbed to the highly virulent H5N1 strain.
She was the sister of two siblings, aged 14 and 15, who also died of bird flu, the first human casualties of the disease outside Southeast Asia and China, where the disease has killed nearly 80 people since 2003.
The ministry said two more patients from the southeastern cities of Siirt and Sanliurfa identified as carriers of the virus were in stable condition. Anatolia news agency said both were children.
The test results brought to 18 the number of Turks infected with the disease, including the three who died, since it emerged in the east of the country last month.
In a brighter development, a 12-year-old boy and a nine-year-old girl, under treatement for H5N1 infection in the north and east of the country repectively, were discharged after they responded positively to treatment, officials said.
WHO warning
At a WHO-sponsored meeting in Tokyo to discuss measures against a possible pandemic, the organisation's director for the Western Pacific issued a somber warning.
"As the new human cases in Turkey show, the situation is worsening with each passing month and the threat of an influenza pandemic is continuing to grow every day," Shigeru Omi told delegates.
The WHO and the British-government-funded Medical Research Council said in a statement in London that an analysis of one of two viruses taken from two flu deaths in Turkey suggested the H5N1 strain was mutating towards a form adapted to humans.
Scientists fear that millions could die in a worldwide pandemic if the virus mutates into a form that would make it communicable among humans.
In Indonesia, the health ministry said a 29-year-old woman had died of bird flu, following the death last week of a 39-year-old man.
If the results are confirmed by the WHO, Indonesia's H5N1 death toll would rise to 13 and the worldwide toll to 80.
In a bid to prevent fresh infections, Turkey's agriculture ministry said it had sent leaflets to all of the country's 81 provinces, informing people about the disease and how it spreads.
It said all national television channels had started broadcasting spot warnings, urging people to stay away from poultry and wash their hands if they come into contact with fowl.
Turkish Agriculture Minister Mehdi Eker on Thursday blamed free-range poultry in rural areas and city outskirts for the spread of the virus to nearly a third of the country's 81 provinces.
"We have to accept this is an important risk factor," Mr Eker said in a televised speech.
He said the deadly strain had so far been identified in poultry in 11 provinces, with suspected outbreaks in 14 others, and that vets had culled some 355,000 wild birds and poultry.
Border checks
Following warnings that the disease may spread to Turkey's neighbours, Russia said Thursday that it was conducting medical checks on airline passengers arriving from Turkey.
Bulgaria warned its citizens to avoid travelling to regions affected by bird flu, including its neighbours Turkey and Romania.
Similar travel advisories against Turkey have been issued by Russia and Britain, but WHO officials have ruled out any danger in coming to Turkey and health experts from the European Union said after a meeting in Luxembourg that there was no need for a heightened travel alert.
The UN Food and Agriculture Organisation had warned Wednesday that the disease risked becoming entrenched in Turkey and threaten to spread to neighbouring Armenia, Azerbaijan, Georgia, Iran, Iraq and Syria.
Iranian and Turkish officials said Iran had closed its border with Turkey near Dogubeyazit, where the deaths occurred, and a report from northern Iran said all poultry in the region along the frontier was being destroyed. Georgia said it was disinfecting all of its border posts.