The World Health Organization's (WHO) European office on Tuesday called for increased vaccination efforts in Eastern Europe, warning that the omicron "tidal wave" was heading eastwards.

The WHO's regional director, Hans Kluge, noted that in the last two weeks, COVID-19 cases had more than doubled in six countries – Armenia, Azerbaijan, Belarus, Georgia, Russia and Ukraine.

"As anticipated, the Omicron wave is moving east. Ten eastern member states have now detected this variant," Kluge said.

The WHO's European region comprises 53 countries, including several in central Asia.

Kluge lamented that vaccine uptake was still relatively slow in parts of the region.

Less than 40% of over-60s have completed their COVID-19 vaccine series in Bosnia-Herzegovina, Bulgaria, Kyrgyzstan, Ukraine and Uzbekistan.

He added that in Bulgaria, Georgia and North Macedonia under 40% of health care workers had received at least one dose of a COVID-19 vaccine.

"I call on governments, health authorities and relevant partners to closely examine the local reasons influencing lower vaccine demand and acceptance, and devise tailored interventions to increase vaccination rates urgently, based on the context-specific evidence," Kluge said.

He stressed that in the face of an "Omicron tidal wave," and "with Delta still circulating widely in the east," now was "not the moment to lift measures that we know work in reducing the spread of COVID-19."

These measures include avoiding crowded locations, wearing masks indoors, improving ventilation and using rapid tests to identify cases early, Kluge added.