GENEVA (Reuters) - 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 the southern hemisphere, where the winter season is beginning, as it could mix with seasonal influenza and mutate in "unpredictable ways," she said.

"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."

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

Chan said in her closing remarks to the WHO's week-long annual congress that countries in the developing world needed to act quickly to improve their monitoring for the virus, which has caused mainly mild symptoms in most patients so far but could cause more serious effects as it spreads.

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

She also stressed there is 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 said 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."