"The conditions for a public health emergency of international concern have been met," WHO assistant director general Bruce Aylward told reporters in Geneva following crisis talks on the virus long thought to be on the road to extinction.
"If unchecked, this situation could result in failure to eradicate globally one of the world's most serious vaccine-preventable diseases," he added.
The UN health agency convened the two-day closed-door emergency talks last week amid concern that the virus, which currently affects ten countries worldwide, was spreading.
Between January and April this year -- usually considered the low season for polio transmission -- three new importations of the virus were detected, from Pakistan to Afghanistan, Syria to Iraq and Cameroon to Equatorial Guinea, WHO said.
"A coordinated international response is deemed essential to stop the international spread," Aylward said.
Polio, a crippling and potentially fatal viral disease that mainly affects children under the age of five, has come close to being beaten as the result of a 25-year effort.
In 1988, the disease was endemic in 125 countries, and 350,000 cases were recorded worldwide, according to WHO data.
Today, the virus is considered endemic in only three countries: Afghanistan, Nigeria and Pakistan.
And last year, 417 cases were detected globally, and so far this year there have been 74 cases, 59 of them in Pakistan, Aylward said.
Although the infection rates remain tiny compared to previous decades, Aylward stressed that until the virus is completely exterminated, "it is going to spread internationally, and it is going to find and paralyze susceptible kids".
"There is always a risk that if the virus is reintroduced to a polio-free area, it could become endemic again," he said, warning that without eradication, "it could become endemic again in the entire world".
WHO was especially alarmed that the recent cross-border spread of the disease came during the traditional low season, warning that the situation could deteriorate as the high season begins in May.
Considering that this happened "in the low season, this poses now a very real threat to the global eradication efforts," said Helen Rees, who serves on the WHO polio research committee.