Scientists need more time to decide whether to mass produce a vaccine against swine flu, the World Health Organization said Thursday, as the number of cases topped 6,000 around the globe.

Acting assistant WHO director-general Keiji Fukuda told journalists that a meeting of experts came to “no big decisions” on Thursday on whether to begin producing a vaccine to the A(H1N1) virus. Production of vaccines against the new virus could disrupt the ongoing production of seasonal flu vaccines, he added.

Marie-Paule Kieny, WHO director for vaccine research, said last week that Thursday’s meeting would decide whether to ask the UN agency to give the go-ahead for large-scale manufacturing of a vaccine. But Fukuda said more meetings were needed to examine the technical process for manufacturers to ready themselves to go into mass production which will “require several weeks.” “It’s not possible to say that there will be a decision by this date, really it’s a painstaking and difficult process,” said Fukuda.

The number of laboratory-confirmed swine flu cases on Thursday topped 6,497 in 33 countries, with Belgium becoming the latest European nation to be hit. Peru also registered its first case. A total of 65 people have died from the disease, most of them in Mexico.

Most of the confirmed cases of swine flu have been in the United States where the number jumped Thursday to 4,298 sufferers in 47 states including the District of Columbia, health authorities said.

New York officials also announced they were shutting down three schools in response to a swine flu outbreak and that one staff member had been hospitalized in serious condition. The three schools, with a total of about 4,500 students, will close Friday and all next week in response to “an unusually high level of flu-like illnesses at those schools,” Mayor Michael Bloomberg said.

So far only four states have been spared the virus — West Virginia, Mississippi, Wyoming and Alaska. But US health officials say they believe the new flu strain will eventually be reported in all 50 states.