More countries announced confirmed or suspected cases of the virus linked to the outbreak in Mexico on Monday. (CBC)The World Health Organization raised its global pandemic alert to Level 4 from Level 3 on Monday, meaning the global health body feels the virus causing the swine flu outbreak can easily transmit between people.

Dr. Keiji Fukuda, the WHO’s assistant director general, confirmed the change during a briefing from Geneva following an emergency meeting of the organization.

Countries should focus their efforts on mitigating the effects of the virus — which the WHO has confirmed has spread to Mexico, Canada, the United States and Spain — because containment is impossible, he said.

However, Fukuda added: "A pandemic is not considered inevitable at this time."

Fukuda said the WHO doesn't recommend closing borders or restricting travel but is encouraging people who are ill to delay travel.

"The deliberations of the emergency committee and the decisions of the director general really reflect a lot of very careful and sober discussion and a number of important considerations," said Fukuda.

Commuters in Mexico City wear surgical masks while riding the subway on Monday. Mexico's government is trying to stem the spread of a deadly strain of swine flu as a new work week begins by urging people to stay home if they have any symptoms of the virus believed to have killed more than 100 people. (Eduardo Verdugo/Associated Press)

Despite the WHO's comments, the European Union and the U.S. have advised against non-essential travel to Mexico because of the outbreak of the virus believed to have killed dozens in Mexico. Canada was expected to issue a similar advisory.

The administration of U.S. President Barack Obama said it is already waging a vigorous campaign of prevention, unsure of the outbreak's severity or where it would show up next.

"We want to make sure that we have people where they need to be, equipment where it needs to be and, most of all, information shared at all levels," Secretary of Homeland Security Janet Napolitano told reporters Monday in Washington.

Napolitano said she is working with Canadian and Mexican officials in a "tri-national" approach to the outbreak. Canada's health minister, Leona Aglukkaq, said she has been in touch with her counterparts in Mexico and the U.S.

Mexico raises death toll

The number of confirmed cases rose Monday to 50 in the U.S., the result of further testing at a New York City school. The WHO has confirmed 26 cases in Mexico, six in Canada and one in Spain. All of the Canadian cases were mild, and the people have recovered.

The Mexican government suspects the virus was behind at least 149 deaths in Mexico, the epicentre of the outbreak, with hundreds more cases suspected.

More than 1,900 people have been hospitalized over what Mexican Health Minister José Angel Cordova Villalobos called "grave pneumonia," but 1,070 have since been released. He said health officials expect the number of new cases to rise.

Its first suspected case of swine flu was detected in the southern state of Oaxaca, Villalobos said Monday, but he added it was too early to identify the cause or geographical source.

The Mexican government has also ordered all schools throughout the country closed until May 6.

The majority of the suspected deaths were between the ages of 20 and 50, Villalobos told reporters in Mexico City.

"We are investigating for confirmation on those cases to see if they are, in fact, flu cases," he said.

Spain, Scotland say flu has hit

More countries announced confirmed or suspected cases of the virus Monday.

Health officials in Britain said two cases have been confirmed in Scotland, but a Canadian woman hospitalized in Manchester, England, tested negative for swine flu. The WHO said it is still awaiting official reports from the U.K. about the Scottish cases.

Scottish Health Secretary Nicola Sturgeon said the two confirmed patients are "recovering well."

In Spain, the first patient, a young man who recently returned from Mexico, is responding well to treatment, Spanish Health Minister Trinidad Jimenez said.

Another 17 sick people are suspected of having the virus, but none of the people under observation is in serious condition, the minister told a news conference in Madrid.

"The situation is under control," Jimenez said.

Two people in Israel who recently returned from Mexico have been quarantined with flu-like symptoms, Israeli health officials said Monday.

Meanwhile, in New Zealand, Health Minister Tony Ryall said 13 people who had just returned from a school trip to Mexico and were showing flu-like symptoms were being tested for swine flu.

Pork imports banned

As more suspected cases are reported, countries around the world are taking action to try to stop the spread of the virus.

Airports in Germany, Singapore, Thailand, Japan, Indonesia, the Philippines, India and Malaysia were to begin screening all airline passengers from North America.

Russia, Hong Kong and Taiwan said visitors returning from flu-affected areas with fevers would be quarantined.

Australian Health Minister Nicola Roxon said pilots on international flights would be required to file a report noting any flu-like symptoms among passengers before being allowed to land in Australia.

China said anyone experiencing flu-like symptoms within two weeks of arrival had to report to authorities.

Several countries, including China, Russia and Ukraine, have also banned imports of pork and pork products from Mexico.

Azerbaijan banned all livestock products from North America while Indonesia and Lebanon announced they will ban most imported pork products.

The U.S. Centres for Disease Control and Prevention says people cannot get the flu by eating pork or pork products.

The term pandemic is a medical description of the way an infectious disease spreads and not an indication of its mortality rate.

Canadian health officials have said flu viruses in general kill about 4,000 people a year in Canada.

With files from The Associated Press