Steps to open up from the coronavirus lockdown, introduced at the start of last week, are to be reversed in the German city of Hamburg.

Mayor Peter Tschentscher said the return to a full lockdown would start on Saturday, after case numbers in the northern port city edged up.

The city — Germany's second-largest — hit the brakes on easing lockdown after three days when its seven-day incidence rate rose above 100 cases per 100,000 residents.

On Friday, the incidence had risen to 108.6 cases. As of Saturday, the rules from before March 8 apply again.

"I fear that the situation will deteriorate further. We are in a strong third wave," Tschentscher said.

"We are dealing with a very contagious variant of the virus," he warned. "We are acting earlier — something we found to be very good in previous waves."

Cologne also toughens curbs

Germany's fourth largest city, Cologne, also announced a tightening of the corona requirements on Friday, although to a lesser extent than Hamburg. The incidence value in Cologne was just above the emergency brake value of 100 for the fifth day in a row.

From Monday, only surgical or FFP2 masks may be worn in outdoor zones where masks are compulsory. Previously, ordinary cloth masks were also allowed in such areas.

There will also be a requirement for a negative coroanvirus test for visits to museums and zoos, as well as hairdressers.

"I hope that we can avert a severe lockdown with our measures," Cologne Mayor Henriette Reker said.

Germany's third wave of infections

The news came as German Health Minister Jens Spahn has said that rising infection rates nationwide meant it may be impossible to ease restrictions in the coming weeks.

The state of Brandenburg, the region surrounding Berlin, also crossed the 100 cases benchmark on Friday.

The more contagious B117 COVID-19 strain is believed to be behind the recent surge in cases across the country.

Chancellor Angela Merkel met with the country's 16 state leaders on Friday to hash out a plan to boost the country's vaccine campaign. On Monday, the leaders will meet again to set new lockdown rules based on the latest developments. 

At the same time, Germany's Robert Koch Institute (RKI) for infectious diseases warned that case numbers were rising at a "very clearly exponential rate."

"The infection process is gaining momentum," RKI Vice President Lars Schaade told reporters on Friday.

Germany behind on vaccinations

The health minister for the city state of Hamburg on Friday welcomed renewed authorization to administer the AstraZeneca vaccine, saying that almost 23,000 vaccination appointments had been rescheduled, the state website said.

By Friday, some 164,775 people in the state had been administered at least one dose, with 75,606 people being fully vaccinated, according to the RKI.

The German government has been under continued pressure to speed up its vaccination drive, especially as cases begin to surge.