Mexico City's airport on April 27, 2009

The World Health Organization has raised its pandemic alert level for the deadly swine flu outbreak, signaling that the infection is spreading among humans in community-level outbreaks.

WHO Officials announced Monday that the alert level has been raised from Phase 3 to Phase 4, which is two steps short of the pandemic phase. The decision follows reports that the outbreak is believed to have killed at least 149 people in Mexico and sickened 1,600 others. 

The WHO took action as U.S. Homeland Security Secretary Janet Napolitano said the United States is responding to the swine flu outbreak as if it were a full epidemic, and that travel warnings to Mexico will remain in place as long as the disease is detected.

Director of Homeland Security Janet Napolitano (right), Dr. Richard Besser (left) at the White House on April 26, 2009

Secretary Napolitano told reporters in Washington that she has been in touch with officials in Mexico to coordinate responses to the outbreak.  Mexico has canceled school nationwide until next week and has issued face masks to residents in an effort to contain the illness. 

Forty cases have been confirmed in the United States, while six have been confirmed in Canada. Spain confirmed its first case Monday, and suspected cases are being investigated in New Zealand, France, Israel and Scotland. Some of the ill recently returned from visits to Mexico.

Governments around the world are urging caution with regard to travel to Mexico, and are screening travelers arriving from affected areas. The U.S. Centers for Disease Control and Prevention says the U.S. government is issuing a travel advisory for Mexico out of an "abundance of caution."

United Nations Secretary-General Ban Ki-moon said the world body is concerned that the virus could cause a new influenza epidemic. He said it is not clear if it would be mild or severe, but he noted with concern that those who died in Mexico were young, healthy adults. 

Separately, the U.N. Food and Agriculture Organization says it is mobilizing teams of experts to determine if the new strain has a direct connection to pigs. The FAO says it is asking its technical staff worldwide to be on full alert and report any flu-like illnesses in swine stocks.

Earlier in Washington, President Barack Obama said the United States is closely monitoring cases of the swine flu in the U.S. He said the situation is cause for concern, but "not a cause for alarm." The United States has declared a public health emergency to allow authorities to spend federal money and release stockpiles of anti-viral medication. Mr. Obama said the declaration was issued as a "precautionary tool."

On Monday, the European Union called for an urgent meeting of EU health ministers, likely to take place Thursday. The EU health commissioner recommended avoiding travel to affected areas.

The WHO says swine influenza, or "swine flu," is a highly contagious acute respiratory disease of pigs, caused by one of several swine influenza A viruses. It says symptoms are generally similar to seasonal flu, but that cases have ranged broadly from mild infections to severe pneumonia resulting in death.

Some information for this report was provided by AFP, AP and Reuters.