The spread of polio is an international public health emergency that threatens to infect other countries with the crippling disease, according to the World Health Organisation.

The health agency described the ongoing polio outbreaks in Asia, Africa and the Middle East as an "extraordinary" situation requiring a co-ordinated international response.

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

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Experts are particularly concerned that the virus continues to pop up in countries previously free of the disease, such as Syria, Somalia and Iraq, where civil war or unrest complicates efforts to contain the virus.

Some critics say the rapid spread of polio could unravel the nearly three-decade effort to eradicate it.

WHO began an emergency meeting with experts on Monday on how to halt the spread of the crippling polio virus across international borders in Asia, Africa and the Middle East. Polio passes easily from person to person and can spread rapidly among children, especially in unsanitary conditions.

"Wild polio virus continues to spread internationally from both endemic and re-infected countries," the United Nations health agency said, adding that the meeting in Geneva would last several days.

Health experts from North America, Europe, Asia, Africa and the Middle East will be asked "to advise on whether the current developments on the spread of polio virus constitute a public health emergency of international concern", it said.

Polio re-emerged in Syria in 2013 for the first time in 14 years.