Dr Hans Henri P Kluge, the World Health Organization regional director for Europe, expressed concern over a resurgence of coronavirus infections on the continent as Europe has seen an increase in weekly cases of Covid-19 for the first time in months as restrictions are eased.
In 11 places, which include Armenia, Sweden, Moldova and North Macedonia, accelerated transmission has led to "very significant resurgence", said Regional Director Dr Hans Henri Kluge.
If left unchecked, he warned health systems would be "pushed to the brink".
"For weeks, I have spoken about the risk of resurgence as countries adjust measures," Dr Kluge told a virtual news conference on Thursday.
"In several countries across Europe, this risk has now become a reality - 30 countries have seen increases in new cumulative cases over the past two weeks.
"In 11 of these countries, accelerated transmission has led to very significant resurgence that if left unchecked will push health systems to the brink once again."
The 11 countries and territories were later identified by the WHO as Armenia, Sweden, Moldova, North Macedonia, Azerbaijan, Kazakhstan, Albania, Bosnia-Herzegovina, Kyrgyzstan, Ukraine and Kosovo.
Dr Kluge said countries such as Poland, Germany, Spain and Israel had responded quickly to dangerous outbreaks associated with schools, coal mines, and food production settings, and brought them under control through rapid interventions.
Despite warning about resurgences, he said the WHO anticipated that the situation would calm down further in the majority of countries over the summer.
"But we have indeed to prepare for the fall, when Covid-19 may meet seasonal influenza, pneumonia, other diseases as well, because ultimately the virus is still actively circulating in our communities and there is no effective treatment, no effective vaccine, yet."
More than 2.6 million cases of Covid-19 and 195,000 deaths have been reported in the WHO's European region, which is expansive, covering 54 countries and seven territories across Europe, the Middle East and Central Asia, according to a BBC report.
Almost 20,000 new cases and more than 700 new deaths are being recorded daily.