The World Health Organization declared an influenza pandemic on Thursday and advised governments to prepare for a long-term battle against an unstoppable new flu virus.

The United Nations agency raised its pandemic flu alert to phase 6 on a six-point scale, indicating the first influenza pandemic since 1968 is under way.

'With today's announcement, WHO moves from an emergency to a longer-term response. Based on past experience, this pandemic will be with us for some months, if not years, to come,' WHO Director-General Dr Margaret Chan said in a letter to staff, a copy of which was obtained by Reuters.

People aged 30-50, pregnant women or people suffering from chronic conditions such as asthma, diabetes or obesity are at highest risk, Chan told a news conference.

The virus has killed 109 people in Mexico, where it was first detected in April before spreading to the rest of the world, prompting the Mexican government to temporarily shut schools and businesses in an effort to slow its spread.

Countries from Australia to Chile to the United States are reporting that the new swine flu virus is 'crowding out' seasonal flu, becoming the predominant influenza strain, she said.

For now the virus was 'pretty stable,' but Chan warned that it could still change into a more deadly form, perhaps mixing with the H5N1 bird flu virus circulating widely in poultry.

'So it is incumbent on WHO and all members to stay vigilant and alert for the next year or two or even beyond,' she said.

Mexican health minister Jose Angel Cordova said on Thursday the virus was under control in Mexico but warned there could be a new spike in cases later this year.

There is also a risk the swine flu could mix with its seasonal H1N1 cousin, which has developed resistance to the main antiviral flu drug Tamiflu, made by Roche AG and Gilead Sciences Inc, Dr. Anne Schuchat of the US Centers for Disease Control and Prevention told a briefing.

The United States has been operating on pandemic status for weeks, with hundreds of thousands of cases and at least 1,000 hospitalizations, Schuchat said.

The virus disproportionately makes younger people sick. Some 57 percent of US cases were among people aged 5 to 24, and 41 percent of those hospitalised were in this younger age group.

H1N1 is active in all 50 states and there are so many cases now that in some areas, patients with specific flu-like symptoms -- a fever above 104 degrees F (40 degrees C), cough or other respiratory symptoms -- are presumed to have the new virus.

WHO reiterated its advice to its 193 member countries not to close borders or impose travel restrictions to halt the movement of people, goods and services, a call echoed by UN Secretary-General Ban Ki-moon.

'We must guard against rash and discriminatory actions such as travel bans or trade restrictions,' Ban told a news conference at UN headquarters.

The move to phase 6 reflects the fact that the disease, widely known as swine flu, is spreading geographically, but does not indicate how virulent it is.

Widespread transmission of the virus in Australia, signaling that it is entrenched in another region besides North America, was one of the key triggers for moving to phase 6.

'We are satisfied that this virus is spreading to a number of countries and it is not stoppable,' Chan said. 'Moving to pandemic phase 6 level does not imply we will see an increase in the number of deaths or very severe cases. Quite on the contrary. Many people are having mild disease, they recover without medicines in some cases and it is good news,' she said.

'Although the pandemic appears to have moderate severity in comparatively well-off countries, it is prudent to anticipate a bleaker picture as the virus spreads to areas with limited resources, poor health care, and a high