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A woman in Hong Kong is seriously ill in hospital with bird flu - the first human case of the disease to be diagnosed in the territory since 2003.

Officials said the 59-year-old fell ill shortly after returning from a visit to the Chinese mainland.

It is not yet clear whether she contracted the potentially fatal disease there or in Hong Kong.

The territory has raised its alert level to "serious", meaning there is a risk of contracting the disease.

The last outbreak of the H5N1 bird flu virus in Hong Kong killed six people in 2003.

Health chief York Chow said there was no sign yet that the virus has been spread between humans and that investigations were focusing on poultry as being the source of the infection.

"But we will be concentrating on people who were in contact with her when she showed symptoms and also when she was in Hong Kong," the AFP news agency quoted him as saying.

"The chances of her catching it is most likely on the mainland, but you cannot rule out Hong Kong," he said.

The woman is reported to have travelled to the mainland with her husband and daughter, and to have visited Shanghai, Hangzhou and Nanjing.

Mr Chow said officials would meet on Thursday to decide what further precautions are needed.

The first time the virus crossed the species barrier between poultry and humans in 1997, every chicken in Hong Kong was culled.

The World Health Organization says more than 500 cases of bird flu have been diagnosed worldwide since 2003. Of those, 302 cases were fatal.