Kemal Onal, the Ankara Governor, told the Anatolia news agency on Sunday that two children and an adult were hospitalised in the capital, while a health official in Corum, 240km east of Ankara, said a five-year-old boy had also tested positive.
Abdullah Kama, the Corum provincial health director, told Anatolia the child was in good condition and that no cases of bird flu had been detected among poultry in the province.
It was not immediately clear whether any of the four were infected with the H5N1 strain of the virus, which can be lethal to humans.
The two children in Ankara are from the town of Beypazari, 100km northwest of the capital, where two wild ducks were found dead from the bird flu in a reservoir two days ago.
Bird flu has infected about 40 poultry farm workers in northern Japan and spread westward in Turkey.
Officials at Japan's National Institute of Infectious Diseases have been testing the blood samples of the workers in Ibaraki prefecture since the first outbreak of bird flu there in June.
Some of them have tested positive for antibodies to H5N2 - a milder strain of bird flu than the deadly H5N1 - in preliminary tests.
The presence of antibodies means that the people were once exposed, said Hiroshi Takimoto, an official of the Health Ministry, on Sunday.
None of them had tested positive for the virus nor developed symptoms of a flu, he said, adding that there was no possibility of the virus spreading from the workers to other people.
There have been no confirmed human cases involving the H5N2 strain anywhere in the world, Japanese officials have said.
Takimoto said that the tests so far suggest that some may
have been infected with the H5N2 strain, but more tests and analysis would be needed.
He declined to say exactly how many tested positive for the antibodies and how many samples had been drawn, saying the tests were continuing and the ministry planned to announce
the results early this week.
Kyodo News agency reported that about 40 people are suspected of having been infected with the H5N2 strain after about 400 people were tested.
There have been several outbreaks of bird flu in Ibaraki, about 100km north of Tokyo.
Bird flu hit Japan in 2004 for the first time in decades.
There has been one confirmed human case involving the H5N1
virus, but no reported human deaths.
Turkey: Spreading west
Meanwhile, in Turkey, dead chickens in two villages in Zonguldak province on the Black Sea coast, about 1200km from the worst-hit eastern areas where the virus killed two children, tested positive for the disease, Yavuz Erkmen, the province's governor, said on Sunday.
Erkmen did not specify whether it was the H5N1 strain, which
can be lethal for humans.
"No people have so far been infected" in the area, he told the Anatolia news agency.
Two children in eastern Turkey
have died from bird flu
In the province of Yozgat, 200km east of Ankara, officials said the virus was detected in dead fowl in at least one village.
The results of tests from three other neighbouring towns were expected soon, they said.
Gokhan Sozer, the governor of Yozgat, told Anatolia that the infected village had been quarantined and all winged animals there slaughtered.
Three people from the village who fell ill after eating sick chicken were hospitalised, Anatolia reported. But Sozer said the patients had caught colds and described them as being in good condition.
The death of two Turkish children from the H5N1 strain of the virus mark the first human fatalities from bird flu outside Southeast Asia and China, where the disease has killed more than 70 people since 2003.
Three siblings from the town of Dogubeyazit, near the border with Iran, died over the past week after coming into close contact with infected chickens.
While two of them were confirmed to have caught the H5N1 strain, the definite cause of the death of the third is not yet known.
Two other children hospitalised in the eastern city of Van were also confirmed to be carrying the virus.