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. 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 registered in the United States where the number jumped Thursday to 4,298 sufferers in 47 states including the District of Columbia, health authorities 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.

Mexico, which has been the epicenter of the outbreak and now has 2,656 confirmed cases, has said it will seek compensation for the damages suffered after fears of the disease drove away thousands of tourists -- a critical source of foreign revenue.

"There are grounds to apply for compensation ... for the country that has been hardest-hit" by the virus, Health Minister Jose Angel Cordova told reporters.

Cordova said compensation for lost tourist revenue could come from the World Bank or the Inter-American Development Bank.
Belgium meanwhile reported a second case of swine flu in the country, the day after announcing it had discovered its first infection.

The WHO also announced that it would shorten its world assembly scheduled to start on Monday in Geneva from nine to four days because of the outbreak.

China, meanwhile, stepped up a search for people who had come into contact with the mainland's two confirmed swine flu patients.

Authorities in the Chinese capital Beijing and eastern Shandong province were looking for plane and train passengers who might have travelled with a 19-year-old student, who on Wednesday became the second confirmed sufferer on the mainland.

China, criticized for its handling of the SARS crisis six years ago, has reacted aggressively to prevent a major new flu outbreak in the world's most populous nation.

The youth apparently felt ill on Sunday, two days after landing in China from Canada, but nevertheless boarded a train on Monday for Shandong province with a fever, sore throat and a headache. Canada has so far the third largest number of swine flu cases and one woman has died there.

The Shandong provincial health department was still looking for more than 20 people who travelled in the same train carriage as the man identified only as Lu, as well as any passengers from his Air Canada flight to Beijing.

Earlier, a 30-year-old man was confirmed to have the virus in the southwestern city of Chengdu. He had been in the United States, where three people have died of the disease, before coming home to China.

Two other cases have been confirmed in Hong Kong. Authorities there said they had quarantined six people who travelled with the second case, a 24-year-old man, by plane from San Francisco.

The cases in China and Hong Kong highlighted concerns that the virus -- believed to be a mix of bird and human flu which came together in pigs -- could spread further around the world as sufferers travelled by air.