GENEVA - The World Health Organization on Monday raised its flu pandemic alert level from three to four, signalling a "significant increase in risk of a pandemic" under its criteria.

Keiji Fukuda, acting WHO assistant director-general for health, security and the environment, told journalists that while the hike is a "significant step towards pandemic influenza, it's also a phase which says we are not there yet."

"A pandemic is not considered inevitable at this time," he added.

Health experts on an emergency pandemic panel met earlier Monday to recommend whether the WHO should raise the alert level as the likely death toll from the swine flu virus rose to 149 in Mexico.

The WHO's two-year-old alert system, which was revised this weekend, has six levels, and level four means the organization sees a "significant increase in risk of a pandemic."

But the WHO reiterated that travel restrictions were not justified and would have little effect, even if under phase four countries should consider screening travellers.

However, Fukuda warned that every region in the world was at potential risk from swine flu given the mobility of people by air travel, repeating a warning the WHO gave during the Severe Acute Respiratory Syndrome (SARS) and bird flu outbreaks in Asia over the past decade.

"I think that in this age of global travel where people move around in airplanes so quickly, there is no region to which this virus could not spread," said Fukuda.

Nevertheless, he stressed that the UN health agency did not recommend closing borders or restricting travel.

"With the virus being widespread... closing borders or restricting travel really has very little effects in stopping the movement of this virus," he said.

"We would have to install very draconian restrictions on travel to have an impact on the movement of this virus," Fukuda added.

Under the WHO's alert levels, in the subsequent phase five there would be human to human spread of the virus into at least two countries in a same region, marking "a strong signal that a pandemic is imminent".

The top level, six, is reserved for a full pandemic, with wider international human to human spread.

However, the emergency committee was cautious in raising the alert level, since although the virus fulfilled criteria about human transmission, it did not fulfill a further one of "widespread" community infection.

"It was clearly felt that we were seeing human-to-human transmission and that put us into phase four," Fukuda explained.

But there was no evidence of widespread infections within a country, and where it had moved abroad from Mexico it was largely with infected people who travelled.

Fukuda said that experts also recommended during the meeting that the virus is currently too "widespread to make containment a feasible" strategy.

As a result, countries should focus on mitigating and preventing swine flu, he said.

The experts also recommended that the WHO "take steps to facilitate the development" of a vaccine against the swine flu virus found in the latest outbreak.

"In general it would take four to six months to develop a vaccine and have the first batch available," said Fukuda.

But the panel stopped short of recommending a complete shift in global vaccine production capacity, warning that it would be "prudent" to continue regular seasonal vaccine production as well.

"The influenza season is just kicking in in the southern hemisphere," Fukuda remarked.

The number of confirmed cases in the United States doubled to 40 on Monday, while Britain and Spain recorded their first swine flu victims.

Mexico said the number of confirmed and suspected deaths from the flu had risen to 149, while some 1,600 people were thought to be infected. Other countries urged against non-essential travel to the tourism hotspot.