"Ukraine has no vaccines," Dorit Nitzan, who heads WHO's country office in Ukraine, said on Tuesday.
"They don't have any vaccines in their storage," Nitzan told reporters in Geneva.
"Even before the crisis they had low (immunization) coverage," she said, noting that there was a dearth of "every kind of vaccine."
The shortage raises deep concerns that polio could break out in Ukraine, Nitzan said, pointing out that the crippling disease that mainly hits young children "usually comes in countries in turmoil."
The conflict in eastern Ukraine has killed more than 2,700 people and forced more than half a million people to flee their homes, according to UN figures.
The fighting on top of Ukraine's already weak health system and dwindling supplies of medicines has created "a looming health emergency," WHO warned.
The UN agency pointed out that some 7,000 people had been injured in the fighting.
At the same time, 32 hospitals are no longer fully functioning — 17 of them have been shelled and damaged — while up to 70 percent of health staff have fled the conflict-hit eastern areas of Donetsk and Lugansk.
The health needs in Ukraine are "immense", Nitzan said.
WHO said it and its partners aim to step up their health services in the country, and plan to provide medicine and medical supplies to some 340,000 people.
In mid-August, WHO had appealed for $14 million (11 million euros) to scale up its Ukraine operations, but has so far received just $40,000, Nitzan said.