The World Health Organization (WHO)'s pandemic alert level remains at phase 5, despite the quick increase of new A/H1N1 infections in Japan, the chief of the UN agency said on Tuesday.

World Health Organization (WHO) Director-General Margaret Chan speaks at a press conference in Geneva May 19, 2009. [Xinhua Photo]

"I must emphasize we are still in phase 5," WHO Director-general Dr. Margaret Chan told a news conference in Geneva, where the 62nd World Health Assembly (WHA) is being held.

The total number of laboratory-confirmed A/H1N1 flu cases in Japan has increased to 159 as of Tuesday, while the number for Sunday was only seven, according to the tally of the WHO.

But Chan said most of the infections in Japan happened in schools through contacts with patients.

There may be some cases that health authorities were not able to identify linkage of patients, but this is not unusual, Chan said, adding that similar situations had already happened in Britain and some schools in New York.

"So we will continue to work with the (Japanese) government and monitor the evolution of the situation," Chan said.

According to the WHO's current pandemic alert system, phase 5 means a pandemic is imminent, while a raise to phase 6, the highest level, will mean a pandemic is under way.

But the changes of alert phases are mainly based on the geographic spread of the flu virus, instead of the severity and deadliness of diseases.

On the opening day of the WHA on Monday, health ministers of Britain, Japan and some other countries urged the WHO to revise the alert system and take into consideration the severity of the disease before declaring a pandemic.

Chan told the assembly that she would follow their instructions carefully, particularly concerning the criteria for a move to phase 6.