The World Health Organization has declared the spread of polio is an international public health emergency.

Outbreaks in Asia, Africa and Middle East are an "extraordinary event" needing a co-ordinated "international response", the agency says.

It recommends citizens of affected countries travelling abroad carry a vaccination certificate.

It says Pakistan, Cameroon, and Syria "pose the greatest risk of further wild poliovirus exportations in 2014."

Polio mainly affects children under five years old.

The virus is transmitted through contaminated food and water, and multiplies in the intestine. It can then invade the nervous system, causing paralysis in one in every 200 infections. It is capable of causing death within hours.

'Ongoing risk'

"The conditions for a public health emergency of international concern have been met," said Bruce Aylward, WHO Assistant Director General.

He was speaking after last week's emergency meeting in Geneva on the spread of polio which included representatives of the affected countries.

"The international spread of polio to date in 2014 constitutes an 'extraordinary event' and a public health risk to other states for which a co-ordinated international response is essential," the WHO's International Health Regulations Emergency Committee said in statement.

"If unchecked, this situation could result in failure to eradicate globally one of the world's most serious vaccine preventable diseases."

The WHO also lists Afghanistan, Equatorial Guinea, Ethiopia, Iraq, Israel, Somalia and Nigeria as "posing an ongoing risk for new wild poliovirus exportations in 2014."

It is only the second time in the WHO's history it has made such a declaration, the first being during the swine flu pandemic of 2009, the BBC's Imogen Foulkes in Geneva reports.

The polio virus is endemic in just three countries - Pakistan, Afghanistan, and Nigeria. But attacks on vaccination campaigns in Pakistan in particular have allowed the virus to spread across borders.

Syria, which was polio free for 14 years, was re-infected with polio virus from Pakistan.

Refugees are still pouring out of Syria, to Jordan, Lebanon and Turkey, and checking whether all of them have been vaccinated will be impossible, our correspondent says.