ANKARA, Turkey - International agencies warned against panic but cautioned Wednesday that Turkey's neighbors may be unable to keep out the bird flu that has infected at least 15 people across the country in the last week.

Nations bordering Turkey continued to step up preventive measures, disinfecting cars at border crossings, handing out leaflets and blocking people carrying birds. But health officials said the measures may prove insufficient.

"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.

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. Three children have died, but only two of those cases were confirmed to have tested positive for bird flu.

Greece was sending 500 veterinary staff to border areas and spraying cars and trucks from Turkey with disinfectant. Bulgaria told its citizens to avoid contact with birds. Syria stepped up checks on entering Turks. Georgian television broadcast images of panicked villagers in a border region killing chickens, geese and ducks en masse.

"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 news conference with Turkey's health minister.

Greece also is gathering enough antiviral drugs to supply 5 percent of the country's 11 million people.

"Whatever is humanly possible is being done. The development of the disease in Turkey is not good. … We must not panic and we must not relax," Greek Health Minister Nikitas Kaklamanis said after an emergency ministers' meeting on bird flu.

Danzon said there were no signs that the deadly strain was being transmitted person to person in Turkey, where all of the cases appeared to have involved people who touched or played with infected birds.

Health experts have been watching closely for indications that H5N1 is mutating into a potent form passed between people, which could cause a pandemic capable of killing millions.

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 TV spots urged people to wash their hands after contact with poultry.

The measures did not keep an Italian consumer group from urging authorities to impose a ban on travel to Turkey.

Asked about whether countries should impose such restrictions, Danzon said there was no reason for such measures.

"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.

EU spokesman Michael Mann praised Turkish efforts to control the diseases spread there, saying "we are happy with the measures that have been taken so far by Turkey."

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

"We don't need to confirm each and every case to confirm the outbreak," said Guenael Rodier, a senior WHO official for communicable diseases.

Rodier said experts were monitoring the progress of H5N1 confirmed in two boys aged 5 and 6 who are hospitalized in Ankara but have not displayed symptoms. Rodier calling the cases the first documented at such an early stage, and indicated that authorities expect the boys to develop symptoms.

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

The group's top official in Asia said East Asia remains at greatest risk for the spread of bird flu, and the world's attention must remain concentrated on improving the region's pandemic preparedness, despite fresh outbreaks elsewhere.

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