MEXICO CITY —  A new swine flu strain that has killed as many as 68 people and sickened more than 1,000 across Mexico has "pandemic potential," the World Health Organization chief said Saturday, and it may be too late to contain the sudden outbreak. The U.S. Centers for Disease Control and Prevention confirmed that the virus has spread widely and cannot be contained, Reuters reported.

"It is clear that this is widespread. And that is why we have let you know that we cannot contain the spread of this virus," Reuters quoted the CDC's Dr. Anne Schuchat.

The World Health Organization's director-general, Margaret Chan, said the outbreak of the never-before-seen virus is a very serious situation and has "pandemic potential." But she said it is still too early to tell if it would become a pandemic.

"The situation is evolving quickly," Chan said in a telephone news conference in Geneva. "A new disease is by definition poorly understood."

Scientists have warned for years about the potential for a pandemic from viruses that mix genetic material from humans and animals. Another reason to worry is that authorities said the dead so far don't include vulnerable infants and elderly. The Spanish flu pandemic, which killed at least 40 million people worldwide in 1918-19, also first struck otherwise healthy young adults.

This swine flu and regular flu can have similar symptoms — mostly fever, cough and sore throat, though some of the U.S. victims who recovered also experienced vomiting and diarrhea. But unlike with regular flu, humans don't have natural immunity to a virus that includes animal genes — and new vaccines can take months to bring into use.