A Turkish teenager whose brother died of bird flu also succumbed to the disease Thursday, a Turkish doctor said, as authorities tried to determine if the siblings had contracted the worrisome H5N1 strain of the virus.

If so, the brother and sister would be the first people to die outside eastern Asia in the latest outbreak of the H5N1 strain. Preliminary tests in Turkey indicated they had the strain, and samples were being sent to a British lab for confirmation.

A senior World Health Organization official said the results of two tests in the brother's case were probably correct.

"It's unlikely we're dealing with false positives," said Guenael Rodier, a special adviser on communicable diseases at WHO's offices in Copenhagen, Denmark. "There's no reason to believe it's wrong."

Fatma Kocyigit, 15, died in a hospital in the eastern city of Van, four days after the death of her 14-year-old brother, Mehmet Ali Kocyigit, the Anatolia new agency reported, citing Ahmet Faik Oner, the doctor who treated the siblings. Doctors suspect their 11-year-old sister also has bird flu and are awaiting the results of tests to confirm that.

Authorities are closely monitoring H5N1, for fear it could mutate into a form easily passed between humans and spark a pandemic.

The virus has killed 74 people — mainly farm workers in close contact with fowl from Vietnam, Thailand and Indonesia — according to the WHO, whose figures were last updated on Dec. 30.

Birds in Turkey, Romania, Russia and Croatia have recently tested positive for H5N1.

The three children were admitted to the hospital last week after developing high fevers, coughing and bleeding in their throats. Eight other patients at the hospital had similar symptoms and were being tested for bird flu, said Huseyin Avni Sahin, head physician at the hospital.

Sahin told private NTV television that other patients with the symptoms were hospitalized in the eastern city of Erzurum because his hospital was not capable of handling more cases.

Several outbreaks among birds have been reported in Turkey's impoverished eastern region, where many people raise poultry. Some 1,500 birds have been destroyed in the area since last week.

Agriculture Minister Mehdi Eker traveled to Van to discuss the situation with local officials Thursday.

"We're facing an important problem," Eker told reporters. "We should calmly take all measures required by science and medicine and implement them without panicking."

The children helped to raise poultry on a small farm in the eastern town of Dogubeyazit, close to Iranian border, and were in close contact with sick birds.

Most of the other eight patients at the hospital were from the same town, some 40 miles away from the town of Aralik where Turkish authorities last week said some chickens had tested positive for an H5 variant of bird flu.

Eker said at least two other outbreaks elsewhere in the region had been confirmed.

In October, more than 10,000 fowl were culled in western Turkey, where the H5N1 virus was detected

Authorities have said the virus was believed to have been brought by birds migrating from Caucasus regions.