British scientists have found a small mutation in the H5N1 strain of bird flu virus isolated from a Turkish child who died of it, the World Health Organisation said yesterday, but medical experts said it was too soon to gauge its significance.

Only two samples from Turkey have so far been analysed at the World Influenza Centre of the National Institute for Medical Research in Mill Hill, north London. A further 15 or more are on their way from Ankara.

Victims of the deadly H5N1 strain have caught the virus from birds. But if it mutates into a form that can be easily transmissible between humans WHO officials fear a pandemic that could kill millions of people.

Michael Purdue, who is overseeing the response to the avian flu outbreak in Turkey on behalf of the WHO, said the mutation now being reported had been spotted before in east Asia - where the disease has killed more than 70 people - in Hong Kong in 2003 and Vietnam in 2005. It was too soon to say what it meant. "It is just the first virus sample," he said.

In a joint statement last night, the WHO and Sir John Skehel, director of the London institute, said: "Research has indicated that the Hong Kong 2003 viruses preferred to bind to human cell receptors more than to avian receptors, and it is expected that the Turkish virus will also have this characteristic."

The virus also bore similarities to samples isolated at a migratory point for birds in Qinghai, western China.

The mutation was in a surface protein, haemagglutinin, which the virus uses to attach to the respiratory tract. Dr Purdue said it was "clearly not a constant change" since it had been seen only twice so far. He said the virus was still evolving.

Turkish officials announced a rise yesterday, from 15 to 18, in the number of human bird flu cases. Two more were reported in the predominantly Kurdish south-east, where the H5N1 strain was detected more than a fortnight ago.

Tests also showed that an 11-year-old girl who died last week was suffering from H5N1. Hulya Kocyigit's brother, Mehmet Ali, 14, and sister, Fatma, 15, also died of the disease last week, bringing the number of confirmed bird flu fatalities in Turkey to three - all of them children.

Across the country health officials continued their mass cull of poultry, with former president Suleyman Demirel handing over 12 chickens as an example.

The two newest human cases were in the south-east provinces of Siirt and Sanliurfa, where authorities began destroying poultry. The health ministry said the two patients had been in contact with chickens, and were in a stable condition.

Avian flu in birds has now been confirmed in 11 out of Turkey's 81 provinces, but with suspected cases in 14 other provinces. The disease appears to have spread remarkably rapidly from the rugged south-east to the Aegean coast.

Turkey's health minister, Recep Akdag, insisted that the country would overcome the outbreak quickly. "The EU and the world will see Turkey put its signature on a great success," he predicted. "The fact that we have handled the affair from the onset with openness and determination is a clear indication."

However, critics say Turkey's reaction to bird flu has been slow and inadequate.