Another Turkish teenager is seriously ill suffering from the deadly HN51 strain of bird flu, the Turkish health ministry revealed tonight.

A teenage brother and sister died earlier this week, and their 11-year-old sister died this morning. Necdet Unuvar, undersecretary in the Turkish health ministry, said that British laboratories had confirmed that Fatma Kocyigit, 15, and her brother Mehmet Ali Kocyigit, 14, tested positive for the virulent H5N1 strain. It was not yet clear whether their 11-year-old sister, Hulya Kocyigit, also had the strain.

However, Unuvar confirmed that another child, Yusuf Tunc, had also tested positive for H5N1 and was currently hospitalized in the eastern city of Van. It was unclear whether Tunc had any connection to the dead teenagers. He was reported to be in serious condition.

The condition of a fourth ill child from the Kocygit family is understood to have improved considerably, and the boy was no longer attached to a respirator, said Huseyin Avni Sahin, the head physician at the hospital in Van where all the children were being treated.

Apart from Tunc, 19 other people were hospitalized in Van with flu-like symptoms. Authorities said tests were still underway to determine whether any of them had contracted bird flu.

Unuvar said more than 5,000 boxes of the antiviral drug Tamiflu were sent to eastern Turkey and five artificial respiration machines were also sent to the hospital in Van, in an effort to attack a virus that appeared to be moving westward from eastern Asia.

The World Health Organization has sent a team to Van, near the border with Iran, to find out whether the children caught bird flu through human to human transmission - signalling the start of an epidemic.

"The first working hypothesis is that the children touched, played with sick chickens and were infected that way," said Fadela Chaib, a WHO spokeswoman. "But they are also going to try to see if we are faced with a first case of human-to-human transmission, which would be the start of a flu epidemic."

Fears that avian flu has taken hold in Turkey are growing after evidence that the disease is infecting birds in at least three areas of the country, hundreds of miles apart.

Fatma and Hulya Kocyigit died despite treatment with Tamiflu. Their brother, Ali Hasan, six, is receiving treatment. The Kocyigit family are from the remote town of Dogubeyazit in the province of Agri, near the borders with Iran and Armenia and about 60 miles south of Aralik, where vets detected the second outbreak last week.

In Van, Mr Sahin, of the Yuzuncu Yil University Hospital, said yesterday that he was expecting another five suspected cases. In Igdir province to the north, six more people were taken to hospital with suspected bird flu.

Mehdi Eker, the Turkish Agriculture Minister, who flew to the region yesterday, said that bird flu had now been identified in five separate places.

Mehmet Ali was buried yesterday by an imam wearing a mask, and lime was spread over his grave. Tests confirming that it was the H5N1 strain were carried out at the World Health Organisation flu laboratory in Colindale, North London. At the same time, tests on samples taken from the birds are being carried out at the Veterinary Laboratories Agency in Weybridge, Surrey.

The extent of the outbreaks in Turkey has dismayed experts. Although the risks were clear, the Turkish authorities have not been able to prevent the spread to people.

This could mean that the extent of the outbreak in poultry in Turkey has been underestimated, or that the virus can jump more easily from birds to people, according to Professor John Oxford, of Queen Mary’s School of Medicine and Dentistry in London. "It is surprising that there are two deaths and a number of people have been infected in what we thought to be a rather small outbreak," he said.

Worried local people went to hospitals in Dogubeyazit, forcing hospital staff to call the police. Doctors said that they were inundated with calls from people saying that they had slaughtered and eaten sick birds. Contrite villagers handed over chickens that they had kept from health officials. Newspaper reports said that the Kocyigit family had moved their chickens into the house for the winter. Doctors at the hospital said that the children had played with the infected chickens’ remains, including the head, after eating the birds.

Turkish officials imposed a ban on the sale and movement of animals in the region. Agriculture officials in Dogubayazit have culled 1,500 fowl and aimed to kill the remaining 2,000 yesterday before burying them outside the town.

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