Two teenage siblings who died of bird flu in Turkey this week were infected with the deadly H5N1 strain of the virus, the first time the strain has killed humans outside East Asia, the U.N. health agency said Saturday.

The World Health Organization said it was sending specialists to Turkey to determine whether the virus was transmitted from person to person.

"The laboratory in the U.K. said that they have detected H5N1 in samples from the two fatal cases," WHO spokeswoman Maria Cheng told The Associated Press.

Cheng said the spread of the disease from East Asia, where it has killed more than 70 people, was "a concern," but the global risk assessment of a human pandemic was unchanged.

The laboratory reported the results Saturday on the tests from a 15-year-old girl and her 14-year-old brother who died earlier in the week, Cheng said. They have yet to complete testing on the samples from their 11-year-old sister, who died of suspected bird flu Friday. A fourth sibling, a 6-year-old boy, was hospitalized.

"Right now these new cases in Turkey, they don't elevate the global risk assessment, so we're still in the same pandemic alert phase that we've been in for the last couple of years," Cheng said. "But it's something that needs to be monitored very closely."

Cheng said WHO specialists are hoping to reach the infected part of eastern Turkey on Sunday to investigate whether the victims were infected by animals or by other humans.

So far H5N1 has been capable in rare cases of transmitting from poultry to humans in close contact with them, but not from human to human. Experts fear that if the virus should mutate to a strain that passes easily from human to human, it could set off a human flu pandemic.

"It's always a concern when we have H5N1 cases, particularly in a region that hasn't previously reported human cases," said Cheng. "That shows us that the virus is still a threat to public health and clearly that it has a capacity to move and to infect humans."

"But at the moment we don't know enough about the situation to tell whether or not the virus has changed in some way. And that's largely the reason the team has gone in," Cheng said, adding officials hoped to get more information on that next week.

The team of five WHO experts arrived in Turkey this week and were supposed to travel Saturday to the city of Van, near the border with Iran, not far from the village where the three children died. But bad weather delayed their departure until Sunday, Cheng said.

The Turkish victims' doctor said the three children probably contracted the illness by playing with dead chickens.

"We'd like to see all these cases investigated rapidly so that we can determine what the mode of transmission was, what the source of exposure was, and whether or not there were any characteristics of the situation that are different from what we've seen in Asia," Cheng said.

She said the cases — the first human H5N1 cases outside China and Southeast Asia — were worrying in part because of the distance.

"It is a jump," she said. "And if you look at how H5N1 has spread in animals, it sort of follows that pattern and implicates the role of migratory birds because we started seeing last year H5N1 being detected in the Ural mountains, in Siberia, Mongolia, Turkey, Romania."

Cheng said the Turkish cases were in a rural region with a lot of poultry farming and where people tend to live in very close proximity to their poultry.

"So that clearly would elevate their risk," Cheng said.

Turkey's number of suspected bird flu cases — people with flu-like symptoms who had recently been in contact with fowl — reached at least 32 Saturday. At least 20 people were hospitalized in Van.

Five people were hospitalized in the southeastern city of Diyarbakir and seven relatives were admitted to an Istanbul hospital.

Turkish Health Minister Recep Akdag said none of the cases outside of Van appeared to be "probable" or "strongly probable" bird flu cases.

In a village on the outskirts of the hometown of the children who succumbed to bird flu, residents gathered outside an Agriculture Ministry building to complain that no one had come to cull their fowl.

"We have sick chickens, we can't touch them," village administrator Hasan Celik said. "No one is coming."

Iran took measures to prevent the disease from spreading, closing down the Gurbulak border gate, some 25 miles from the dead teenagers' hometown, to private citizens except for Turks and Iranians wanting to return to their own countries. Officials disinfected trucks going into Iran.

Hunting of all birds was banned until further notice across the country.