Hong Kong - A 7-year-old girl has become the third child to die of a mystery illness in Hong Kong, government officials said Friday.

The girl, who was admitted to hospital on August 8 with encephalitis or brain inflammation, died Friday.

Two other girls, aged 9 and 3, died last week with myocarditis, inflammation of the heart, and blood poisoning.

A fourth child, a 3-year-old boy, is also in a critical condition with sepsis and encephalopathy - blood poisoning and brain abnormalities.

Health officials are already conducting an investigation into the cause of the illnesses, however it is not yet known whether one single virus or different viruses are responsible.

In March, the government forced the closure of all Hong Kong primary schools and kindergartens after the deaths of two children aged 3 and 7, caused panic among parents who remembered the grim days of the SARS outbreak which killed 299 and infected 1,755 people.

It brought the return of the "face-mask" which were sold by the millions and worn on the streets, in schools and in workplaces during the outbreak in Spring 2003.

However, an investigation revealed the deaths were not the result of a virulent virus but down to strains of the H1N1 and H3N2 Brisbane virus which has been circulating in different parts of the world earlier this year.

As well as SARS, Hong Kong saw the world's first modern-day outbreak of bird flu in 1997 when the virus infected 18 people, killing six of them. (dpa)