A leading microbiologist urged the British public not to panic as bird flu crept closer to central Europe, with the deaths of three children in Turkey.

A farm boy died after developing pneumonia-like symptoms at the weekend, one sister died yesterday and another aged 11 died early today. A third sibling is also suspected of having bird flu, amid reports that 25 people have been admitted to hospital in connection with the outbreak.

If the deaths are confirmed as being from the H5N1 strain, they would be the first fatalities in the current bird flu outbreak outside south-east Asia, where at least 70 people have died. Samples are being sent to the Veterinary Laboratory Agency near Weybridge, Surrey, for tests.

Professor Hugh Pennington, president of the Society of General Microbiology, said: "This shows we have to keep our vigilance up but the picture, as far as the human population is concerned, hasn't changed. It's still an avian virus and hasn't mutated to enable human-to-human infection. So the message is: don't panic ... If you see a dead bird, stay well away from it and report it immediately to a vet."

The Government echoed the academic's "don't panic" message and said it would monitor the situation in Turkey closely.

Professor Pennington, of Aberdeen University, continued: "If the virus does mutate, it could go through the human population like a dose of salts. But the fact it hasn't done so yet suggests it may not be able to do so as it's been around a long time."

Poultry workers have been urged to minimise risks by maintaining good hygiene, isolating any birds that may be sick and alerting authorities.

The Turkish teenager, Muhammet Ali Kocyigit, 14, died in a hospital in the south-eastern Van province on Sunday. Hulya Kocyigit, 11, died only a day after her 15-year-old sister, Fatma Kocyigit, succumbed to the disease yesterday.

Soldiers have been called in to help cull birds in villages where the virus has been found.