'The key to the public health response is surveillance,' said Peter Cordingley, spokesman for the WHO Western Pacific region in Manila.

'If we do actually get to the cases with antivirals early on, the health outcome is a lot better.'

WHO experts were arriving in Pakistan to try to sort out cases reported there, South Asia's first human infections.

Four brothers and two cousins fell ill last month in Abbotabad, a small city north of Islamabad, while three others who slaughtered poultry in the same area and a nearby town tested positive for the H5N1 bird flu virus earlier this month.

Two of the brothers died, but specimens were collected from only one.

The cases were positive for H5N1 in initial government testing, but WHO will conduct further analysis to confirm the results.

Not ruling out human-to-human
WHO spokesman Gregory Hartl in Geneva said on Sunday human-to-human transmission had not been ruled out, but added that poultry outbreaks had earlier been reported in the area and it was unclear whether all the patients were exposed to sick birds.

A doctor who cared for the brothers also experienced mild flu-like symptoms, but more testing needs to be carried out to determine if she was infected, Mr Hartl said.

With fresh poultry outbreaks reported in a number of countries, including Germany over the weekend, WHO urged nations to be vigilant in identifying and reporting cases in both birds and humans.

Myanmar's first human case was reported Friday in a 7-year-old girl who fell ill last month and survived, while Indonesia, the country hardest hit by the virus, has announced its 93rd death from the virus.

Two human cases were also recently confirmed in China, one of whom died.

The H5N1 virus often flares during the winter months. In some countries, like Indonesia, poultry outbreaks and human cases are reported year round, but many countries experience a flurry of activity when temperatures drop.

'It starts to pop at this time of the year, not just in this region where it's endemic, but it starts to appear in the West,' Mr Cordingley said.

'Between now and April is a very dangerous time of the year.'

At least 208 people have died from the virus, which began plaguing Asian poultry stocks in late 2003. It remains hard for people to catch, but scientists worry it could mutate into a form that spreads easily among people, potentially sparking a pandemic.

Most human cases have been linked to sick birds, but experts suspect limited transmission may have occurred in a few cases involving blood relatives. -- AP