Europe's rollout of vaccines against COVID-19 is "unacceptably slow," World Health Organization (WHO)'s European head Hans Kluge said in a statement on Thursday.

"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 said.

To date, only 10 percent of the region's population have received one vaccine dose, and 4 percent have completed a full course, he added.