MEXICO CITY -- World health officials admitted they were powerless to halt the spread of swine flu and ratcheted up their pandemic alert level as the number of affected countries rose sharply on Tuesday.

As Mexico, epicenter of the outbreak, said 152 people were now believed to have died from the virus, the number of known cases in the US more than doubled while six other countries said they had confirmed their first casualties.

Authorities in Asia -- where memories of the Severe Acute Respiratory Syndrome (SARS) outbreak in 2003 remain fresh -- tried to contain the spread of the virus, screening air passengers from affected areas and advising against non-essential travel to Mexico.

Their counterparts in the United Arab Emirates, one of the world's premier flight hubs, also placed their airports under strict surveillance to spot anyone arriving who might have swine flu.

But the World Health Organization (WHO) said border screenings "don't work," while the EU's health commissioner said there was no need for travel restrictions.

"Border controls do not work. Screening doesn't work," WHO spokesman Gregory Hartl said in Geneva.

"If a person has been exposed or infected... the person might not be symptomatic at the airport," he said. "We learn as we go on. SARS was a huge learning experience for all of us."

The WHO raised its flu pandemic alert level from three to four on Monday night -- signaling a "significant increase in the risk of a pandemic."

Keiji Fukuda, acting WHO assistant director-general for health, security and the environment, said that given the widespread nature of the virus, all corners of the world are at potential risk.

"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," Fukuda said.

The outbreak was too "widespread to make containment a feasible" strategy, he added.

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.

Fukuda's line was echoed by the European Union health commissioner Androulla Vassiliou who said that while precautionary measures were advisable, "at this juncture I don't see any point on restricting traveling."

But reflecting the split in how to deal with the outbreak, France joined Britain in advising against all but essential travel to Mexico.

The sense of gloom was compounded by predictions from leading experts that a pandemic was now all but inevitable.

"It is very likely we are at the beginning of a pandemic. We are near there," said Yuen Kwok-yung, head of microbiology at the University of Hong Kong.

And Dmitry Lvov, head of the Russian Academy of Sciences' Institute of Virology, said: "The risk of a pandemic in the world is very high."

The Mexican government meanwhile said the probable national death toll from the virus had risen to 152, while the number of cases under observation reached 1,614.

Faster and more effective laboratory tests for the flu were to begin Tuesday, Health Minister Jose Angel Cordova said.

"We're in the decisive moment of the crisis. The number (of deaths) will continue rising," Cordova said.

Mexico City, home to around 20 million people, remained eerily quiet with the capital's zoos, museums, churches, courts and many restaurants closed.

British tour operators, including Thomas Cook, halted holidays to Mexico after two people who returned to Scotland from a honeymoon in the resort town of Cancun fell ill with the swine flu virus.

Spain on Tuesday also reported a second confirmed case of swine flu and said 32 other people are suspected of having contracted the disease after visiting Mexico while suspected cases were being probed in Germany and Austria.

A total of 44 people are thought to have been infected in the United States while Canada has six cases.

And in a further illustration of its global reach, Israel and New Zealand both confirmed their first swine flu casualties.

In Asia, Thai medical authorities placed a woman in quarantine in hospital while South Korea investigated a "probable" victim. Australia meanwhile probed 70 possible cases.

WHO officials in China said they were investigating several people with suspicious symptoms, but played down the chances that any were likely infected.

"Swine flu is an international problem now, it's crossed two continents, it's got to be dealt with by international agreements," said British Prime Minister Gordon Brown as his government called a meeting of its COBRA crisis cell.