LONDON — Raising a worldwide alarm, the World Health Organization announced Monday that spread of polio is an international public health emergency that could grow in the next few months and unravel the nearly three-decade effort to eradicate the crippling disease.

The agency described current polio outbreaks in Asia, Africa and the Middle East as an “extraordinary event” that required a coordinated international response. It was the first-ever international alert on polio.

“Until it is eradicated, polio will continue to spread internationally, find and paralyze susceptible kids,” Dr. Bruce Aylward, who leads WHO’s polio efforts, said during a press briefing.

Polio usually strikes children under five and is most often spread via infected water. There is no specific cure, but several vaccines exist.

Experts are particularly concerned that the virus continues to pop up in countries that were previously free of the disease, such as Syria, Somalia and Iraq — where civil war or unrest complicates efforts to contain the virus. That spread has happened during the traditionally low season for polio spreading, leaving experts worried that cases could spike in the coming months.

Last week, WHO convened an emergency committee to decide whether the current polio outbreaks — in at least 10 countries in Asia, the Middle East and Africa — merit the declaration of an international health emergency.