Health experts are expressing growing concerns about the increasing number of COVID-19 cases in Europe, as the continent's "third wave" threatens to destabilize recovery plans.

Several regions are reporting spikes in localized cases, with lockdown fatigue and sluggish vaccine rollout threatening to place a strain on already stretched hospitals once again.

In Germany, almost 13,000 new coronavirus infections were reported on Friday. That represents a 20 percent increase compared to the same day the week prior.

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The head of Germany's Robert Koch Insitute, Lothar Wieler, said the situation was worsening.

"We are now at the beginning of the third wave and together, we need to keep it as low as possible. That's only doable if we prevent infections and if we prevent case numbers from exploding again," he said.

The sentiment was echoed by German health minister Jens Spahn, who said the surge in cases threatened to prolong lockdown measures.

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"The situation remains tense, the number of cases is on the rise again, slowly, but it's on the rise and variants are spreading. So we will have to prepare for a few more very challenging weeks," he said.

The outlook is also deteriorating in France. Health Minister Olivier Veran has signaled additional restrictions could be placed on Paris, as cases in the capital climb. 

France's intensive care admissions are also now at a three-and-a-half-month high as more infectious variants of the virus spread.

Some hospitalized patients in the northern Hauts-de-France region have started to be transferred to Belgium, with the cross-border operation beginning as Belgian hospitals also feel the strain from growing numbers. 

But despite the pressures, Belgian health officials say they remain ready to help their neighbors.

"Everyone is tired, everywhere it is the same, but we don't really have a choice," said Delphine Deneir from the AZ West Hospital in Veurne. "People are sick and it is our job to help them, so we will keep trying to do the best we can." 

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The threat of a third wave follows growing tensions over Europe's vaccination rollout.

Austrian Chancellor Sebastian Kurz said new analysis showed certain EU countries had received more vaccine doses than their per-capita allocation.

"It appears that while the heads of state and government made decisions, another committee, a so-called steering board of health officials, made different agreements with pharmaceutical companies, which is clearly in contradiction to the agreement," he said.

"There are indications that there was some kind of bazaar, where additional agreements were made between member states and pharmaceutical companies."