Most of those stricken with acute flaccid paralysis, a symptom of diseases including polio, in Deir al-Zor province are children under the age of two, WHO spokesman Oliver Rosenbauer said. More than 100,000 children under the age of five are deemed at risk of polio in the eastern province.
There is no cure for the highly infectious disease, it can only be prevented through immunisation, usually three doses.
"The main concern right now is to quickly launch an immunisation response," Rosenbauer said. Vaccination campaigns are being planned across Syria from November but the logistics were still being discussed, he said.
"Everybody is treating this as an outbreak (of polio) and is in outbreak response mode," Rosenbauer said.
The WHO, a U.N. agency, said on Saturday that two suspected cases of polio had been detected, the first appearance of the disease in Syria since 1999.
Initial tests came back positive for polio in two of the 22 cases and final laboratory results due next week from a WHO reference laboratory in Tunisia are "very, very likely" to confirm presence of the virus, Rosenbauer said.
Most of the 22 victims are believed never to have been vaccinated or to have received only a single dose of the oral polio vaccine.
With about 4,000 refugees fleeing Syria's civil war daily, polio immunisation campaigns are also planned in neighbouring countries, where there may be gaps in coverage, he said.
Polio invades the nervous system and can cause irreversible paralysis within hours. It is endemic in just three countries, Nigeria, Pakistan and Afghanistan, but sporadic cases also occur in other countries.
Asked whether the virus may have been imported into Syria by a foreign fighter, Rosenbauer said: "The first step is virological verification that it is the polio virus. The next step is that every isolated virus gets looked at genetically to see where is the parent. Hopefully that will provide some clarity on where it would have come from."
Worldwide, cases of polio decreased from an estimated 350,000 when the campaign began in 1988 to 223 reported cases in 2012, according to the WHO. So far this year, not including the cases in Syria, there have been 296 cases worldwide.
The United Nations' Children's Agency (UNICEF) said on Thursday it had chartered a plane filled with vaccines and food to combat the rising threat of other types of disease and malnutrition among Syrian children.
"Hospitals visited by UNICEF staff are reporting an upward trend in the number of children being admitted with moderate and severe acute malnutrition compared to two years ago," a UNICEF statement said.
The United Nations says more than 100,000 people have been killed in Syria's 2-1/2-year conflict, more than 2 million Syrians have fled the country and millions more have been displaced inside Syria. The fighting has caused a sharp deterioration in services and infrastructure and many people are trapped in areas of fighting in unsanitary conditions with little food or medical supplies.