LONDON -- The World Health Organization says it's worried that millions of doses of polio vaccine might be wasted in Ukraine after a patient group raised concerns that the doses are unsafe.

The U.N. health agency announced last month that the crippling disease had been identified in Ukraine for the first time in nine years and warned that low immunization rates put the country at high risk of an outbreak.

Polio is a potentially fatal water-borne disease that mainly strikes children and has been brought under control in all but two countries in the world.

The all-Ukrainian Council for Patients' Rights and Safety says the 3.7 million doses of oral polio vaccines donated by the United Nations partially thawed out on the plane to Ukraine and now shouldn't be used. The government is considering this week what to do with them.

According to WHO guidelines, the vaccine must be kept frozen while being stored, although thawing and freezing are normal parts of transporting the doses. Sanofi Pasteur, the vaccine's maker, said the temperature changes reported do not affect the vaccine's potency.

"There is doubt about the quality of the (vaccines)," said Viktor Serdyuk, president of the Ukrainian patients group that already has a working relationship with WHO.

Serdyuk said he wasn't sure what should be done with the vaccines but options include destroying them. The Ukrainian health minister did not respond Thursday to requests for comment.

WHO's European director Zsuzsanna Jakab insisted the vaccines are "absolutely safe" and said it's crucial that a planned vaccination campaign start next week to protect Ukrainian children from the paralyzing, potentially fatal disease.

Although only two cases of polio have been detected in Ukraine so far, there are likely hundreds of other asymptomatic children spreading the virus. The region where the cases were found also borders Romania, Hungary, Slovakia and Poland.

Jakab said she is concerned that vaccine rumours might lead the health ministry to suspend immunization efforts.

"If the ministry of health is not empowered to start vaccinating, then we need to reach a higher level of government," Jakab said. "If the vaccination campaign doesn't start on Monday, then I will get on a plane and go to Ukraine personally to speak to the president."

Jacobsen reported from Moscow.