Turkey reported five new human cases of the H5N1 strain of bird flu in preliminary tests Monday, a Health Ministry official said.

The tests were conducted in Turkish labs on samples provided by five people suspected of having the disease, said the official, speaking on condition of anonymity because she was not authorized to speak to the media.

The new cases raise the number of human bird flu cases in Turkey to 15. They have not yet been confirmed by the World Health Organization.

Health Ministry official Turan Buzgan said the new cases were discovered in four separate provinces in eastern and central Turkey, as well as on the Black Sea coast, indicating the disease was continuing to spread across the country.

Health officials note the virus so far has only been confirmed in humans who were in close and prolonged contact with birds. But they are watching the disease's spread and development for fear it could mutate into a form easily transmitted between humans, sparking a pandemic.

The European Union took further steps Monday to contain the disease, agreeing to ban imports of untreated feathers from six nations bordering eastern Turkey.

The ban on imports from Armenia, Azerbaijan, Iraq, Iran, Georgia and Syria is expected to come into force after formal approval Tuesday by the European Commission.

Other poultry imports from those countries are already banned, said Michael Mann, spokesman for the EU's head office.

In Turkey, 10 people had previously tested positive for H5N1 in tests done in the country's labs, four of which were confirmed by the World Health Organization.

Those four include two siblings who died last week in the eastern city of Van — the first confirmed fatalities caused by the virus outside eastern Asia, where 74 people have been killed by H5N1 since 2003. A third sibling also died in Van of bird flu, but the WHO lab has yet to confirm H5N1.

On Monday, Health Minister Recep Akdag visited with the father of the children in Dogubayazit, a largely Kurdish town near Van where most of the bird flu cases have originated. He was accompanied by officials from WHO and the European Union.

The doctor who treated the siblings said they probably contracted the illness by playing with dead chickens.

Earlier Monday, authorities reported 18 additional people were hospitalized in southeast Turkey while undergoing testing for bird flu.

Three new cases of H5N1 were reported Sunday in Ankara — two young brothers and a 65-year-old man — as well as two other cases in Van, about 600 miles to the east.

The boys — Muharrem Canak, 5, Iskender, 2 — apparently caught the virus while playing with gloves their father had used to handle two dead wild ducks outside Ankara, their doctor Metin Dogan said. The boys did not appear sick, despite testing positive for the virus in tests done at Turkish labs, Dogan said.

Health officials believe the best way to fight the spread of bird flu is the wholesale destruction of poultry in the affected area. But they often run into problems in rural areas like Dogubayazit, where villagers have resisted turning in their birds.

On Sunday, a group of Turkish workers in the town had to climb over a wall when a woman refused to open the door and hand over several chickens, insisting they were not sick. The workers said they would return with police.