GENEVA, Feb. 4 (Xinhua) -- The World Health Organization (WHO) confirmed on Sunday that the death of a Nigerian woman last month was caused by the H5N1 strain of bird flu.

    In a statement, the UN agency said "initial positive test findings from a laboratory in Nigeria" had been confirmed by a WHO collaborating center in London.

    Samples from all contacts of the 22-year-old woman, who died in the Nigerian capital of Lagos on Jan. 17, have tested negative, said the statement, which was posted on the WHO website.

    The WHO said further investigations were now under way to identify the source of the infection.

    The Nigerian government first announced on Wednesday that the young woman had died after catching the H5N1 virus from infected chicken. It was the first human bird flu death in the country.

    Nigeria, Africa's most populous nation, was the first on the African continent to detect the H5N1 virus in poultry. The virus has spread quickly over the past year despite a series of controlling measures.