Tests have confirmed that at least two of the three Turkish teenagers killed by bird flu had the deadly H5N1 strain.
An 11-year-old girl died on Friday, days after her brother and sister. Another brother is still being treated.
The cases are the first human deaths from bird flu outside South-East Asia, where more than 70 people have died of the H5N1 strain.
Tests in a UK laboratory confirmed on Friday that two of the children and one other patient did have the H5N1 strain.
The children's family kept poultry at their home in Dogubeyazit, close to the Iranian border in Turkey's eastern Van province.
The latest victim, Hulya Kocyigit, has been buried beside her 14-year-old brother Mehmet Ali Kocyigit, who died on Sunday, and their sister Fatma, 15, who died on Thursday.
Their brother was said to be showing signs of improvement.
All four children developed symptoms including a high fever, coughing and bleeding in the throat.
Doctors said they had been playing with the heads of chickens who had died of bird flu.
Turkey fights back
The hospital in Van is treating more than 20 other people, one of whom is in a critical condition, according to a doctor there.
The town where the family lived has been placed under quarantine and no people or animals are allowed to move in or out of the district, according to the World Health Organization (WHO).
Turkey is carrying out a mass cull of poultry in an effort to contain the virus. Some 3,500 birds have been culled so far in the Van region and extra supplies of Tamiflu medicine have been sent.
"We don't expect a pandemic or anything like that in Turkey but there is a real risk for people who are in close contact with fowl," said Health Minister Recep Akdag.
Experts from the WHO and the EU have been sent to Turkey to help them deal with the outbreak.
Turkish Agriculture Minister Mehdi Eker says at least four new outbreaks of bird flu in poultry have been confirmed in the eastern provinces of Igdir and Erzurum and the south-eastern province of Sanliurfa.
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