"Vaccines present our best way out of this pandemic... However, the rollout of these vaccines is unacceptably slow" and is "prolonging the pandemic", WHO director for Europe Hans Kluge said in a statement, AFP reported.
"We must speed up the process by ramping up manufacturing, reducing barriers to administering vaccines, and using every single vial we have in stock now," he added.
The organization said that Europe's virus situation was "more worrying than we have seen in several months".
Five weeks ago, the weekly number of new cases in Europe had dipped to under one million, but "last week saw increasing transmission of COVID-19 in the majority of countries in the WHO European region, with 1.6 million new cases", it said.
The total number of deaths in Europe "is fast approaching one million and the total number of cases about to surpass 45 million", it said.
The WHO's European region comprises 53 countries and territories and includes Russia and several Central Asian nations.
The organization warned that the rapid spread of the virus could increase the risk of new variants of concern developing.