Mexico City, Apr. 28: The World Health Organization has raised the alert level in relation to the `Swine Flu’ epidemic to Phase 4, meaning there is sustained human-to-human transmission of the virus causing outbreaks in at least one country.

On Monday, the alert was raised above Phase 3.

Putting an alert at Phases 4 or 5 signals that the virus is becoming increasingly adept at spreading among humans.

Phase 6 is for a full-blown pandemic, characterized by outbreaks in at least two regions of the world.

World health officials are now racing against time to extinguish a new flu strain that is jumping borders The U. S. is prepared for the worst even as President Barack Obama has reassured Americans that it is being contained.

With the swine flu having already spread to at least four other countries, authorities around the globe are like firefighters battling a blaze without knowing how far it extends.

"At this time, containment is not a feasible option," said Keiji Fukuda, assistant director-general of the World Health Organization, which raised its alert level on Monday.

At the White House, a swine flu update was added to Obama''s daily intelligence briefing. Obama said the outbreak is "not a cause for alarm," even as the U. S. stepped up checks of people entering the country and warned U. S. citizens to avoid nonessential travel to Mexico.

"We are proceeding as if we are preparatory to a full pandemic," said Homeland Security Secretary Janet Napolitano.

The European Union health commissioner suggested that Europeans avoid nonessential travel both to Mexico and parts of the United States. Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus.

Mexico, where the number of deaths believed caused by swine flu rose by 50 percent on Monday to 152, is suspected to be ground zero of the outbreak. But Mexican Health Secretary Jose Angel Cordova late Monday said no one knows where the outbreak began, and implied it may have started in the U. S.

Worldwide there were 79 confirmed cases, including six in Canada, one in Spain and two in Scotland. Thirteen are suspected in New Zealand, and one is suspected in both France and Israel.

Symptoms include a fever of more than 100, coughing, joint aches, severe headache and, in some cases, vomiting and diarrhea.

In a bid to prevent mass contagion, Mexico canceled schools nationwide until May 6, and the Mexico City government is considering a complete shutdown, including all public transportation.

Richard Besser, the CDC''s acting director, said his agency is aggressively looking for evidence of the disease spreading and probing for ways to control and prevent it.

The best way to keep the disease from spreading, Besser said, is by taking everyday precautions such as frequent hand washing, covering up coughs and sneezes, and staying away from work or school if not feeling well.

WHO spokesman Peter Cordingley singled out air travel as an easy way the virus could spread, noting that the WHO estimates that up to 500,000 people are on planes at any time.

Governments in Asia — with memories of previous flu outbreaks — were especially cautious. Singapore, Thailand, Japan, Indonesia and the Philippines dusted off thermal scanners used in the 2003 SARS crisis and were checking for signs of fever among passengers from North America. South Korea, India and Indonesia also announced screening. (ANI)