Geneva - World Health Organization Director General Margaret Chan raised the pandemic alert for swine flu by one level to phase 5 Wednesday night in Geneva.

The declaration of Phase 5 is a strong signal that a pandemic is imminent. It is characterized by human-to-human spread of the virus into at least two countries in one WHO region.

"Based on an assessment of all available information and following several expert consultations, I have decided to raise the influenza pandemic alert from phase 4 to phase 5,," Chan announced at WHO headquarters.

WHO did not rule out that it may yet up the level once more, to the highest-possible phase 6, which would mean that a global pandemic was underway.

Chan said all countries should activate pandemic flu plans, which had been prepared following the avian flu outbreak earlier this decade, and called on governments and health authorities to be on high alert for influenza-like illnesses and pneumonia.

WHO said the decision to make the transition to phase 5 was based on the geographical spread of the disease as well as the fact that evidence supported sustained human-to-human transmission at the community level.

"We are seeing this in Mexico, and as information emerges from the US authorities we are also seeing that there," Chan said.

"A pandemic by definition means we have established transmission in multiple countries, multiple regions of the world," explained Keiji Fukuda, WHO chief of health security.

"We are seeing it now in two countries in one region," he added, referring to the United States and Mexico.

"If this continues we expect it to be in other countries and regions," Fukuda noted, saying the WHO was "in the process of moving towards" phase 6, but added that "we are not there yet."

While phase 6 would mean that the new outbreak had taken on global dimensions, it would not indicate how severe the disease had become.

"A pandemic virus is precarious, unpredictable and will take us by surprise," Chan said.

Numerous times during her press conference, Chan stressed that the nature of the virus was still not yet fully understood, including how severe the disease could get. She said the virus was showing signs that it could range from very mild to quite severe.

"Let me say it is important to take this very seriously, to maintain vigilance and to track the virus as it evolves," Chan said, but cautioned that "we need to maintain a level of calmness so we will continue to manage this in a rational manner."

There was no need for border closures, restrictions on movement of people, good or services, and to stop eating properly cooked pork products.

However, there was still need for awareness, also at an individual level, and WHO officials reminded people to maintain proper hygiene and limit some physical contact, such as greeting friends with hugs and kisses.

On Monday the UN agency raised the pandemic alert level from phase 3 to phase 4. (dpa)