NEW YORK - NEW York announced on Friday the closure of three more schools in response to spreading swine flu, bringing the total to six, as an assistant principal at one school remained critically ill.

Health and school authorities announced two new closures in the borough of Queens and one in Brooklyn 'after documenting unusually high levels of influenza-like illnesses'. The schools were closed for five days.

On Thursday, officials shut down three other schools in Queens after outbreaks of what was believed to be swine flu.

An assistant principal at one of the schools closed Thursday remained 'hospitalized in critical condition,' Mayor Michael Bloomberg said on Friday.

Health Commissioner Thomas Frieden, who is about to take up a new post as director of the US Centres for Disease Control and Prevention (CDC), noted that symptoms remained mild.

However, he said that the virus appeared to move faster than with ordinary flu - possibly because it is a new virus.

It 'appears to be spreading more rapidly than traditional influenza,' he said. 'We don't know why that is, but the fact that we have no vaccine or experience of being infected with this influenza are likely factors.' 'We are not seeing more severe illness than we see in regular flu season. What we are seeing is larger clusters.'

The World Health Organisation said Friday that the disease is continuing to spread worldwide.

Latest WHO data showed 7,520 people in 34 countries confirmed to have caught the A(H1N1) virus - 1,000 more than a day ago.

According to the figures, most of the deaths have occurred in Mexico, with five in the United States and one each in Canada and Costa Rica. -- AFP