The World Health Organization has found a link between the outbreak of polio in Syria, which has so far paralyzed around 13 children, and a strain of the virus from Pakistan.
The vaccination of children in Syria has been disrupted by the 31-month conflict between President Bashar al-Assad's forces and rebels fighting to topple his regime.
"Thirteen cases of wild poliovirus type 1 (WPV1) have been confirmed in the Syrian Arab Republic," the WHO said in a statement.
"Closely related wild poliovirus strains have also been detected in environmental samples in Israel, (the) West Bank and Gaza Strip since February 2013."
The United Nations said last week that emergency plans were being made to vaccinate more than 20 million children in the Middle East after polio resurfaced in Syria.
The vaccinations would be carried out over six months in Egypt, Iraq, Jordan, Lebanon, the Gaza Strip and West Bank, Syria and Turkey.
The initiative was announced after the WHO reported that polio had re-emerged in Syria for the first time in 14 years.
The UN health agency launched a large-scale campaign on October 24 to immunise 1.6 million children in Syria against polio, measles, mumps and rubella.
"Given the current situation in the Syrian Arab Republic, frequent population movements across the region and the immunisation level in key areas, the risk of further international spread of wild poliovirus type 1 across the region is considered to be high," said the WHO.
"A surveillance alert has been issued for the region to actively search for additional potential cases."
An estimated 120,000 people have been killed in Syria's civil war, which has also displaced millions of people from their homes.