Surging coronavirus figures across Europe should serve as "a wake-up call", the World Health Organization's regional director has said.
Hans Kluge said in the past two weeks the number of new cases had doubled in more than half of European member states.
"We have a very serious situation unfolding before us," he said.
He was speaking as the number of confirmed infections worldwide crept towards the 30 million mark.
More than 942,000 people have died since the coronavirus emerged in China late last year.
Speaking in Copenhagen on Thursday, Mr Kluge said 300,000 new infections were reported across Europe last week alone and weekly cases had exceeded those reported during the first peak in March.
"Although these numbers reflect more comprehensive testing, it also shows alarming rates of transmission across the region," he told reporters.
Mr Kluge said the figures "should serve as a wake-up call for all of us".
Hospital admissions and deaths have not yet seen a similar rise, although Spain and France are seeing an upward trend.
While younger people - who are less likely to be severely impacted if they are infected - currently make up the largest proportion of newly recorded coronavirus cases, there are fears of many more cases of serious illness if the virus spreads to older and more vulnerable groups.
According to the WHO, there have been five million confirmed cases and more than 228,000 fatalities across Europe since the pandemic began.
'We can fight it again'
Mr Kluge told Thursday's briefing that lockdown measures introduced in the spring and early summer had produced clear results.
"In June cases hit an all-time low," he said. "The September case numbers, however, should serve as a wake-up call for all of us."
But he stressed that knowledge about the best ways to contain the virus had improved since the pandemic first spread to Europe.
"We have fought it back before and we can fight it back again," he said.
Earlier this week Mr Kluge warned that Europe should prepare for a rise in the number of coronavirus deaths in October and November, warning: "It's going to get tougher."
What's the situation around Europe?
In the UK, the number of new daily cases has reached the highest level since mid-May. New rules have been imposed to limit social gatherings and further local restrictions for parts of north-east England were announced on Thursday.
France, meanwhile, recorded 10,593 cases on Thursday - the highest daily number since the pandemic began.
Health Minister Olivier Véran acknowledged the coronavirus "is again very active"; Lyon and Nice, two of France's biggest cities, have been given until Saturday to come up with new measures to tackle their outbreaks.
Mr Véran and Prime Minister Jean Castex are among several ministers facing legal action over their handling of the crisis.
Other countries are seeing their own spikes:
- Spain on Wednesday recorded 239 new coronavirus deaths, the highest number since June. Many of the country's new infections have been in the capital Madrid, where authorities are planning to announce new lockdown measures
- The Czech Republic has reported more than 2,000 daily cases for the first time. The country's prime minister called on people to follow the rules to prevent an exponential increase in infections
- Cases have also reached a new daily peak in the Netherlands, with the government expected to announce new coronavirus measures on Friday