Concerned over the rapidly spreading swine flu, the World Health Organization (WHO) has upped the influenza pandemic alert to phase 5, just one step short of declaring a bona fide global pandemic.

"All countries should immediately activate their pandemic preparedness plans," said WHO Director-General Margaret Chan, calling on government ministries and manufacturers of vaccines and antiviral meds to mobilize resources immediately to deal with the rapidly evolving swine flu outbreak.

The decision to raise the alert level from phase 4 to phase 5 means there is evidence of sustained human-to-human transmission that is capable of causing community outbreaks in two countries in a single WHO region, Chan told reporters in a teleconference this afternoon. That’s now true in the Americas region, which includes Mexico and the U.S., both of which have had such transmission.

So far there have been 148 reported cases of swine flu in nine countries -- 91 (incuding one death) in the U.S., 26 (including seven deaths) in Mexico, 13 in Canada, five in the United Kingdom, four in Spain, three in Germany, three in New Zealand, two in Israel, and one in Austria -- according to the WHO. But media reports suggest Mexico has accumulated as many as 2,400 cases, over 150 of them fatal.

Will swine flu will escalate into a full-fledged global pandemic, prompting the WHO to raise the alert level to phase 6?  For this to happen, the new virus must take root in multiple countries in multiple regions of the world, said Keiji Fukuda, the WHO's assistant director-general for health security and environment.

And that may not be far off, Fukuda suggested: "We think that we are in the process of moving toward there." But he provided no timeline.

Still, Chan noted, "influenza viruses are notorious for their rapid mutation and unpredictable behavior." The epidemic could remain mild and eventually grind to a halt, she said, or go in the opposite direction, causing widespread severe illness.

See our in-depth report for more on the swine flu outbreak.