The World Health Organization on Wednesday tried to allay fears of a massive bird flu outbreak in Turkey, telling people not to panic but urging them to avoid contact with sick or dead poultry.

Preliminary tests in the last week indicate that 15 people in Turkey have been infected with the deadly H5N1 strain — the largest number of cases in a single week since late 2003, when the virus began sweeping Asia. Two Turkish children have died.

"The worst situation is a panic situation. There is no reason to panic," Dr. Marc Danzon, WHO regional director for Europe, told reporters at a joint press conference with Turkish Health Minister Recep Akdag. Danzon said health officials were doing "everything that is known to maintain and manage this difficult situation."

Meanwhile, in Rome, the U.N. agriculture agency warned Wednesday that the Turkish outbreak could spread to neighboring countries.

"The virus may be spreading despite the control measures already taken," said Juan Lubroth, senior animal health officer at the U.N. Food and Agriculture Organization.

"Far more human and animal exposure to the virus will occur if strict containment does not isolate all known and unknown locations where the bird flu virus is currently present."

Danzon said there were no signs that the deadly strain was being transmitted person to person. Health experts have warned of the possibility that H5N1 could mutate into a potent form easily passed between people, triggering a pandemic capable of killing millions.

WHO said earlier Wednesday that two more people sickened by bird flu in China have died, bringing the total number of humans killed by the disease in that country to five and pushing the death toll worldwide to 78.

Asked about whether countries should ban or restrict their citizens from traveling to Turkey, Danzon called it a "non-story" and said there was no reason in his view to take such measures.

In Turkey, all of the cases appeared to have involved adults or children who touched or played with infected birds.

"The people of the country need to perfectly understand that the danger is contact between sick or dead poultry and a human being, especially a child," Danzon said. "This is the key point for the future. This is where we need to pass messages to the population and inform local leaders."

European governments, scrambling to avoid the specter of a mutation that could trigger a pandemic, sprayed trucks from Turkey with disinfectant. In Italy, a consumer group urged the government to impose a ban on travel to Turkey, and in Greece, veterinary inspectors stepped up border checks.

Underscoring the vulnerability neighboring countries feel, Bulgaria began issuing its citizens special instructions on how to deal with an outbreak.

Turkey's government, anxious to demonstrate to its citizens and the European Union that it was taking decisive action, ordered more than 300,000 fowl destroyed as a precaution. Authorities also distributed leaflets in eastern regions most affected by the outbreak, cautioning people not to touch fowl, while television spots urged people to wash their hands after contact with poultry.

Health officials said Tuesday that most of the 70 or so people hospitalized with flu-like symptoms had tested negative for bird flu.

"The situation has been taken seriously from the beginning" in Turkey, Danzon said.

WHO officials said initial investigations suggest there is no change in how the disease is spread, and experts are hoping there may be some differences in the behavior of poultry farming families in Turkey to explain the high number of cases. Another possibility is a change in the virus. Tests were under way, officials said.

WHO so far has confirmed only four of Turkey's 15 reported cases as H5N1, but said it is confident the remaining samples would be positive.