The EU health agency ECDC on Wednesday warned that vaccinations alone would not stop the rise of the Omicron variant of the novel coronavirus, and said "strong action" was urgently needed.

"In the current situation, vaccination alone will not allow us to prevent the impact of the Omicron variant, because there will be no time to address the vaccination gaps that still exist," Andrea Ammon, director of the European Centre for Disease Prevention and Control (ECDC), said in a statement.

The agency meanwhile also raised their risk assessment for Omicron's impact on public health from "high to very high" to "very high."

The agency called for strengthening and reintroduction of "non-pharmaceutical interventions," such as mask-wearing, hand hygiene, distance working and prevention of crowds in public spaces.

"It is urgent that strong action is taken to reduce transmission and alleviate the heavy burden on health care systems and protect the most vulnerable in the coming months," Ammon added.

Ammon said that "there are indications that community transmission is already ongoing in EU/EEA countries," and that modelling suggested that a further rapid increase in Omicron cases was "imminent."

World Health Organization (WHO) chief Tedros Adhanom Ghebreyesus said Tuesday that the Omicron variant had been reported in 77 countries and had "probably" spread to most nations undetected "at a rate we have not seen with any previous variant".

Tedros also cautioned against "dismissing Omicron as mild," pointing out that even if the variant does cause less severe disease, "the sheer number of cases could once again overwhelm unprepared health systems".