The World Health Organization (WHO) says it's worried that millions of doses of polio vaccine might be wasted in the Ukraine after concerns were raised by a patient group that the doses are unsafe.
The oral polio vaccine must be kept frozen while being stored, according to WHO guidelines.
The all-Ukrainian Council for Patients' Rights and Safety says the vaccines partially thawed out on the plane to Ukraine and now shouldn't be used - a proposition being considered this week by the Ukrainian government.
WHO's European director Zsuzsanna Jakab said the vaccine is "absolutely safe" and that it's crucial a planned vaccination campaign start next week to protect Ukrainian children from the paralyzing disease.
Polio was identified in the Ukraine last month, the first time in nine years.