GENEVA - Countries should be ready for more serious H1N1 flu infections and more deaths from the newly discovered virus, World Health Organization chief Margaret Chan said on Friday.

The highly contagious strain must be closely monitored in parts of Asia, Africa and South America where the winter season is beginning in case it mixes with seasonal flu and mutates in unpredictable ways, Chan told the closing session of her United Nations agency's annual congress.

In cases where the H1N1 virus is widespread and circulating within the general community, countries must expect to see more cases of severe and fatal infections, she said. We do not at present expect this to be a sudden and dramatic jump in severe illness and deaths.

In the WHO's latest tally, the strain has infected more than 11,000 people in 42 countries and killed 86.

Chan told the end of the week-long World Health Assembly that poorer countries need to quickly improve their monitoring for the new flu, which has caused mainly mild symptoms in most patients but could become more serious as it spreads.

This is a subtle, sneaky virus, she said. We have clues, many clues, but very few firm conclusions.

Another senior WHO official, Keiji Fukuda, later told a news conference the United Nations agency was rethinking its criteria for declaring a full H1N1 pandemic is underway to factor in its severity as well as its geographical spread.

What we are looking for and what we will be looking for is something, events, which signify a really significant increase in risk of harm to people, said Fukuda, acting WHO assistant director-general.

The WHO's global flu alert is currently at level 5 out of 6, meaning that a pandemic is imminent.

Officials from the WHO's 193 member states have been pushing this week for an altered definition of a Phase 6 pandemic to reduce the risk of unnecessary public panic.

The bottom line here is we are trying to walk a very fine line between not raising panic but also not becoming complacent, Fukuda said, adding that criteria would be reviewed now that the assembly had ended.

The WHO's current rulebook requires a full pandemic to be declared once the air-borne virus is spreading in a sustained way in two regions of the world. The H1N1 strain has been most prevalent in North America and has also caused growing pockets of infection in Japan, Spain and Britain.

Chan said there was little real difference between the WHO's current pandemic alert level of 5 and the highest of 6 in terms of preparedness measures taken, and she would consult experts before opting to raise it again.

The decision to declare an influenza pandemic is a responsibility and a duty that I take very, very seriously, she said. I will consider all the scientific information available. I will be advised by the emergency committee.

The WHO expects that by the end of May, laboratories will be able to send candidate viruses to drug companies to be tested as vaccine viruses, according to Fukuda.

We are hopeful that by the end of June, by the beginning of July, this will be the time when commercial companies will be in the position of beginning to, or being able to make H1N1 vaccine, he said.

While the new flu virus dominated their meeting, officials also passed resolutions on the need to improve treatment of multi-drug-resistant tuberculosis and prepare for a surge in diseases including mosquito-borne malaria due to global warming.