Mehmet Ali Kocyigit (left) and his family lived on a poultry farm

A 14-year-old boy who died last weekend was found to have the disease, despite earlier results indicating otherwise.

His sister died in eastern Turkey early on Thursday and a third sibling is in hospital with bird flu symptoms.

Although tests are still being carried out, a senior official from the World Health Organization says it is likely the boy had the deadly H5N1 strain.

Guenael Rodier, a special adviser on communicable diseases at the WHO, told the BBC that while he was not surprised by the case, it was a significant development.

"It shows a geographic extension of human cases and certainly increases altogether the overall likelihood that at some point this virus will adapt to humans," he said.

"So it's important this is going to be investigated, and a WHO team is on its way to the Far East and Turkey with nationals to investigate."

Mutant fears

The WHO has said that while the virus is spreading, the cases are not the start of a pandemic, as it was not passed between humans.

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The Turkish Health Ministry says both teenagers who died tested positive for the H5N1 strain of the virus. A British laboratory is expected to confirm those results in the next few days.

At least 14 other people are being treated as suspect cases.

In the two years since bird flu was detected in Asia, around 70 people have died from the H5N1 strain.

There are no confirmed cases however of human-to-human transmissions, and sufferers are believed to have contracted it from close contact with sick birds.

But health experts have expressed fears that it could mutate, becoming a virus that could spread among humans like common influenza.

Turkey reported its first cases of bird flu among poultry on 8 October, which were confirmed as the H5N1 strain. Thousands of birds were culled in and around Kiziksa, western Turkey, where the outbreak occurred.

'Don't panic'

The boy, named as Mehmet Ali Kocyigit, died on Sunday in the city of Van in eastern Turkey. His sister, Fatma, died in the early hours of Thursday.

This is not the start of the pandemic

Dr David Nabarro

World Health Organization

The family lived and worked on a poultry farm in the town of Dogubayezit, close to the border with Iran.

Health Minister Recep Akdag said the family kept infected birds in their home.

"There are two cases that have been confirmed as positive by the laboratory, said Mr Akdag.

"Another case is suspected of being positive. We have a pandemic plan ready. There is no need to be too alarmist."

Murat Akova, of Ankara's Hacetepe University, said close contact with poultry was the likely cause of infection.

Migration path

The agriculture ministry is conducting an emergency cull of poultry in the region, instructing farmers to take extra precautions. The health ministry says it has enough medicine in stock to cope.

Dr David Nabarro of the WHO urged caution among millions fearful of a global pandemic.

"This is not the start of the pandemic. The pandemic starts when there is human to human transfer, confirmed and sustained," he said.

In Europe, the H5N1 strain was discovered in bird flocks in Turkey, Russia, Romania and Croatia, but had not previously spread to humans.

Turkey lies on the migration path of wild birds suspected of spreading the flu westwards from Asia.