The World Health Organization (WHO) has received reports of 1,003 confirmed cases of A/H1N1 influenza from 20 countries, WHO Director-General Margaret Chan said Monday.

"Nearly everybody in the world is susceptible to infection. It is this almost universal vulnerability to infection that makes influenza pandemic so disruptive," she told a special meeting of the UN General Assembly on the disease via a video link.

"Large numbers of people falling ill can be highly disruptive to economies and to the functioning of routine medical services," she said.

Chan said the world is still at phase five of the pandemic alert level,  which means the need "to maintain high-level vigilance and monitor further international spread of the virus and further spread in countries that are already reporting cases."

"We do not know how long we will have till we move to phase six, which indicates that we are in a pandemic. We are not there yet," she said.

But she added that the criteria will be met when another region outside North America shows "very clear evidence of community level transmission."

Students wear masks in Bogota May 4, 2009. Colombia on Sunday reported its first confirmed H1N1 flu virus case, a man who recently returned from Mexico but who remains in good condition at home after tests conducted in the United States came back positive.

Recalling the experiences of past influenza pandemics, Chan warned that the initial situation can change "in many ways, with many, many surprises."

"Historically, influenza pandemic has encircled the world in two, sometimes three waves," she said, citing the deadliest 1918 pandemic, which killed tens of millions, as an example.

The 1918 pandemic began in a mild way and then evolved into "a far more deadly one." Its first wave was so mild that its significance as a warning signal was missed, she said.

"The world today is much alert to such warning signals, and much better prepared to respond," she noted.

"At this point, we have no indication that we are facing a situation similar to that in 1918," she said. "This situation can change, simply because influenza virus are constantly changing in unpredictable manners."

"The only thing can be said with certainty about influenza viruses is that they are entirely unpredictable, no one can say right now how the pandemic will evolve, or indeed whether we are going into a pandemic," she said.

Chan urged countries to make rational responses at a time of economic downturn, and refrain from introducing measures that are "economically and socially disruptive, yet have no scientific justification and bring no clear public benefit."

Global manufacturing capacity, though greatly increased in recent years, is still not sufficient to produce enough anti-viral and pandemic vaccines to protect the entire world population in time, she said.

she also called for "global solidarity" to respond to the challenge of the influenza, in particular to ensure that developing countries are not left without protection.