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The spread of the potentially fatal Middle East respiratory syndrome has become more serious and urgent, the World Health Organization said Wednesday, according to CNN.

At the same time, WHO said that -- for now, at least -- the illness known as MERS does not constitute a global health emergency

Declaring an emergency is "a major act" that can "raise anxieties," said Dr. Keiji Fukuda, the organization's assistant director-general for health security.

Despite concerns about the syndrome, researchers have not found "any increasing evidence of person-to-person transmissibility," he said.

There have been 571 confirmed cases of MERS, including 171 deaths, according to the World Health Organization. The number of countries with confirmed cases expanded to 18, with a case in the Netherlands, WHO reported Wednesday.

Many of the cases are in Saudi Arabia and the United Arab Emirates.

Even without any official worldwide alert, Anne Schuchat, the head of the CDC's National Center for Immunization and Respiratory Diseases, acknowledges that "this is a relatively new virus that does have a high fatality rate," ample reason to pay attention. Authorities haven't pinned down all the details about how exactly it arose and how it spreads, though Schuchat said, "we don't have evidence right now that this is airborne ... the way the measles virus is."

"We don't know as much as we would like so far," the CDC official and assistant surgeon general in the U.S. Public Health Service told CNN's Brooke Baldwin on Wednesday. "... But we're tracking it and trying to understand."