The Robert Koch Institute (RKI) said on Friday evening that the R-number, which estimates how many people the average infectious person passes the virus on to, had risen back up to 1.01.
An R-value of 1.01 means that 100 infected people mathematically infect 101 more people. This could indicate that the more contagious virus variants are spreading despite the lockdown.
"We may be at a turning point again. The downward trend of recent weeks is apparently no longer continuing," RKI head Lothar Wieler said.
The change in the trend was also reflected in the overall number of new cases, with health authorities in Germany reporting 9,164 new infections to the RKI. That was close to 10 percent more than last Saturday.
In addition, 490 new coronavirus-related deaths were reported within 24 hours.
There has been little change in new infections and seven-day incidence in recent days - despite the ongoing severe lockdown.
Wieler said on Friday that the proportion of virus variant B 1.1.7, which conservative estimates suggest is 35 percent more contagious, is rising rapidly in Germany.
In the northern town of Flensburg, the so-called British variant has already gained the upper hand.
In the city on the Danish border, almost all new infections are with the variant that first appeared in the UK, said town mayor Simone Lange. Flensburg has become one of the corona hotspots nationwide.
Denmark has now closed several smaller border crossings to Germany. Important crossings such as Frøslev, Kruså and Padborg are to remain open but with "much more intensive" control, Copenhagen confirmed.
In Flensburg itself, tightened rules have been in place since midnight. As of Saturday, a nightly curfew has come into force between 9 pm and 5 am.
In view of the dangers posed by the viral mutations, Labour Minister Hubertus Heil warned against relaxing the restrictions too soon.
“The federal and state governments must work together to develop a sensible opening concept,” the SPD politician told Redaktionsnetzwerk Deutschland (Saturday). "This must be so safe that we do not have to close everything again after a few weeks."
“All measures must be accompanied by a good testing strategy, and we must keep an eye on the vaccination rollout,” Heil said. “Just because we're all fed up with the lockdown, we can't end it recklessly.”