U.S. swine flu patients may have transmitted the virus to others in the United States, the World Health Organization said Tuesday, indicating that the new strain is spreading beyond travelers returning from Mexico.

The global health agency said so far most people confirmed with swine flu were infected in Mexico. But the source of some infections in the United States, Canada and Britain is unclear, said WHO spokesman Gregory Hartl.

Hartl said the agency was "looking at the United States" to see if people were passing on the disease there. WHO calls this "community transmission" and says it's a key test for gauging whether the spread of the virus has reached pandemic proportions.

"This virus is still very much in its early days. We're still at the point of trying to figure out how deadly it could be," Hartl told a news conference in Geneva. "If the virus is an efficient virus, if it spreads easily from human to human, it will probably continue to spread."

Hartl said WHO was waiting for U.S. authorities to announce that a number of students at a New York high school have passed the virus on to one another after their return from a spring vacation in Mexico.

"I think we might have one other instance in the U.S.," he said.

Pressed by reporters to elaborate, he declined, saying it was up to U.S. authorities to provide further information.

Possible scenarios include students getting infected who did not travel to Mexico, or students who traveled there but became infected only after returning to the United States, or family members getting infected from returning students.

Only Mexico has reported deaths linked to the disease.

In Canada, some of the six people clearly were infected in Mexico, Hartl said. He called these "imported" infections, but said other cases were still being investigated.

How the British pair became infected is still unclear.

"At least one of the two is imported," Hartl said. "Apparently this was a couple that was in Mexico, in Cancun, on honeymoon. So did one infect the other? Did both get infected in Mexico? We don't know."

WHO has set up a round-the-clock emergency room in Geneva to examine all rumored and confirmed reports of swine flu infections in humans.

Cases confirmed in Britain, Israel and New Zealand are not yet part of the WHO's global tally of 73 confirmed cases because national authorities have yet to relay all information to the world body.

Proof that the virus is spreading among humans outside Mexico could prompt WHO to further raise its pandemic alert level — an indicator of how close the world is to experiencing an outbreak threatening the entire planet.

WHO on Monday increased the alert level from 3 to 4 — out of 6 — after consulting with flu experts from around the world. Its influenza chief Keiji Fukuda warned that "at this time containment is not a feasible option," rejecting calls for a travel ban or other restrictions on Mexico or the United States.

Hartl said WHO is advising countries to aggressively combat outbreaks where they occur.

"Border controls do not work. Travel restrictions do not work," he said, recalling the 2003 SARS epidemic that killed 774 people, mostly in Asia, and slowed the global economy. "There was much more economic disruption caused by these measures than there was public health benefit."