One victim is a 5-year-old boy who fell critically ill after killing a duck for New Year's dinner. Another is his 12-year-old sister, who died last weekend. Three were siblings who succumbed in rapid succession, leaving their grieving parents with just one surviving child.

Just as in East and Southeast Asia, where at least 77 people have died of bird flu since 2003, children appear to be particularly susceptible to the lethal H5N1 strain — if only because they are more likely to touch or play with diseased birds.

"So far it looks like the same pattern," World Health Organization spokeswoman Maria Cheng told The Associated Press Tuesday.

Cheng said the U.N. health agency did not yet have figures for the number of children who have contracted H5N1 worldwide and declined to offer an estimate. But in Turkey, WHO had a clearer picture: All but two of the 21 confirmed human cases, it said, have involved children and teens aged 4 to 18.

Turkey's Health Ministry said Tuesday that preliminary tests had detected the deadly strain in yet another child, who was hospitalized Jan. 12 in the eastern city of Erzurum. It said the 4 1/2-year-old, whose gender was not immediately released, came from Dogubayazit — the hometown of all four of the children who so far have died.

Officials said samples from the child, who was in intensive care, were being sent to a WHO laboratory in Britain for independent confirmation.

Experts fear the virus could mutate into a form spread easily among humans, triggering a pandemic capable of killing millions. The WHO has stressed it has no evidence of person-to-person infection in Turkey, and another agency spokeswoman, Cristiana Salvi, said the mortality rate in Turkey was 20 percent — substantially lower than the 58 percent fatality rate that had been seen in Asia.

Salvi cited early detection and treatment as the likely explanation, but cautioned: "There could be other factors which we are investigating, as a lot of cases are still in the hospital. We need to know how the cases progress."

Health authorities noted that chickens, geese and turkeys often run free in yards where children play, and that even if youngsters don't touch the birds, they can become infected through contact with contaminated bird droppings.

"I appeal to citizens: Please do not get into close contact with your fowl, and especially keep your children away," Health Minister Recep Akdag said Tuesday in an address to parliament.

Doctors in the eastern city of Van were closely monitoring a 5-year-old boy — also from Dogubayazit, near the border with Iran — who was reported in critical condition after contracting H5N1 by killing a duck for dinner on Jan. 1.

The boy, Muhammet Ozcan, was being treated for an infection spreading in his lungs and was receiving oxygen, the Anatolia news agency quoted the hospital's chief physician, Dr. Huseyin Avni Sahin, as saying. His sister, Fatma, died Sunday after being rushed to the hospital bleeding from her mouth. The two had slaughtered the duck together and became ill three days later, WHO said.

A team of U.S. influenza experts met Tuesday with WHO and Turkish health officials in the capital, Ankara, and planned to decide later whether to travel to other afflicted areas. The U.S. Centers for Disease Control also sent experts to Turkey to help combat the disease.

Prime Minister Recep Tayyip Erdogan urged Turks to be "careful and cool-headed" and reassured the nation that everything was being done to contain the crisis.

Later Tuesday, Erdogan met with Zeki Kocyigit, the father of the three siblings who died, and promised to pay for the education of their only surviving child, 6-year-old Hasan Ali. He said the family had decided to move to Ankara.

With Turks complaining of symptoms still checking into hospitals, there were concerns the virus might still be spreading to people despite the precautionary slaughter of nearly 1 million domestic fowl.

Among those getting treatment were three children with bird flu symptoms in Istanbul, where Europe and Asia meet at the Bosporus Strait. Officials were waiting to see if tests confirmed that they, too, were infected with H5N1, which would bring the virus in humans right to Europe's doorstep.