Hong Kong ordered more than half a million primary and kindergarten students to stay home from school for two weeks starting Thursday after three schoolchildren died amid recent outbreaks of the flu.

The government asked one of the territory's top scientists to investigate the three deaths. But the World Health Organization said Thursday there was no sign that the situation in Hong Kong was anything but a regular seasonal flu outbreak.

The outbreak has not been linked to bird flu, which has been detected in birds in Hong Kong. Bird flu remains difficult for humans to catch, though scientists fear the virus that causes it could eventually mutate into a form that spreads easily among humans.

The new flu outbreak raised memories of the deadly severe acute respiratory syndrome, or SARS, which surfaced in southern China in November 2002 and later killed 299 people in Hong Kong. That disease ground this bustling financial capital to a halt as locals avoided going out, while the government was criticized for not responding quickly enough.

The government has ordered all kindergartens, primary and special education schools closed for two weeks starting Thursday, Health Secretary York Chow said. It was the first time the government has closed schools for public health reasons since SARS.

But some students and their parents missed the late Wednesday announcement and went to school anyway.

At the Cho Yiu Catholic Primary School, nine students arrived early Thursday. School staff took their temperature and gave them masks. The students read picture books instead of attending class.

"I'm worried that I'll get infected. Many classmates have been infected with the flu. I'm afraid," said 9-year-old Winnie Ting, who was already wearing a mask before arriving at school.

Principal Chan Kwok-ming said on average 20 students took sick leave every day in recent days.

The schools had been expected to start their Easter Holiday in a little over a week, though the date varies with each school. Chow said bringing the holiday forward would help reduce cross infection among school children and calm public fears.

Explaining his decision on Thursday, Chow said he acted quickly because "when you wait until you have all the data, it's too late."

He said he was particularly concerned that two of the three dead children were treated at the same hospital, and that children may be especially vulnerable to the disease.

Chow has also asked Yuen Kwok-yung, who studied SARS, to head a panel of experts to study the deaths.

The closure will affect nearly 560,000 students at 1,745 schools, according to enrollment figures from the 2006-2007 academic year.

Banker Edmond Lai, 46, said he was washing his hands more frequently and avoiding crowded places — but wasn't worried about a larger outbreak.

"After the experience of SARS, the government's response must have improved," he said.

There was not sign yet of the major public panic that followed the SARS outbreak five years ago. Residents have not started wearing masks en masse as they did during SARS.

In the neighboring Chinese city of Shenzhen, health officials have not recorded a jump in flu cases, Ma Hanwu, deputy head of the Shenzhen Municipal Center for Disease Control and Prevention was quoted as saying in the Shenzhen Special Zone Daily newspaper.

Since March 6, health officials have recorded nine confirmed flu outbreaks and 65 suspected ones in Hong Kong, mostly at schools, affecting 532 people in the territory of nearly 7 million.

WHO spokesman Gregory Hartl said the first child who died tested negative for all types of influenza viruses. The second child tested positive for the flu but also suffered from an underlying metabolic disease. The third — a 7-year-old boy who died Tuesday — had both the flu and encephalitis.

Another WHO spokesman, Peter Cordingley, said Thursday from Manila that "this is just regular seasonal flu."

"There is nothing exceptional about what's happening in Hong Kong," he said, noting the territory has suffered worse flu outbreaks in recent years.