NEW YORK, May 15 (AFP): New York City authorities closed Friday three schools following a swine flu outbreak as the World Health Organisation (WHO) reported more than a 1,000 new cases around the globe in 24 hours.

The WHO said there are now 7,520 confirmed cases around the world, just a day after pleading for more time to decide whether to mass produce a vaccine.

New York officials said an assistant principal of one of the schools that will stay closed through next week was in serious condition in hospital.

There are about 4,500 students at the three schools and city mayor Michael Bloomberg said they were ordered closed because of "an unusually high level of flu-like illnesses."

The A(H1N1) outbreak came less than two weeks after New York health authorities pronounced the all clear after other schools were affected by the virus through students who had visited Mexico.

Bloomberg said swine flu had been identified in four students at one of the schools, all in the Queens district, "as well as in a staff member at the school who is critically ill.

"More than 50 students have been sent home from the school with flu-like symptoms," he said. At a second school, 241 students reported absent Thursday, and at a third 29 students were documented with "influenza-like symptoms."

National health authorities earlier said that the number of confirmed cases of swine flu in the United States jumped Thursday from 3,352 to more than 4,298.