Mexican authorities shut schools across the country and tried to follow up with families of swine flu victims in a bid to stop the virus' spread, as the probable death toll rose to 152.

The Mexican outbreak has sparked global concern as rising cases are detected around the world, and the World Health Organization has raised its flu pandemic alert level from three to four, signaling a "significant increase in risk of a pandemic."

The outbreak has prompted fears that the world may finally be experiencing the flu pandemic that health officials have been warning about for years.

Officials have confirmed 20 people died from the disease, while the number of cases under observation in Mexico has risen over 1,600.

Adding to Mexico's woes, a moderate 5.6-magnitude earthquake rocked the country shortly before noon (1700 GMT) Monday, causing panicked residents to run out on Mexico City streets earlier cleared by flu fears.

"All education activities across the country are suspended from next Tuesday to May 6," Cordova said during a news conference during the quake.

Schools in Mexico City, the heart of the epidemic, were already closed last Friday.

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

As travelers returned from trips to Mexico, Europe's first cases were confirmed in Britain and Spain.

The United States and Britain also advised against non-essential travel to Mexico.