Although gas supply in Germany remained secure, the situation was "tense and a deterioration of the situation cannot be ruled out," BNetzA said on Monday.
In the event of a shortage in winter, the government would have to decide on who gets no more gas, a "political nightmare scenario," Minister for Economic Affairs and Climate Action Robert Habeck warned last week.
"That will put Germany to a test that we have not had for a long time," Habeck said.
The Nord Stream 1 pipeline, which went into operation in 2011, transports gas from Vyborg in Russia to Lubmin in northeastern Germany. From there, gas is further transported to other European countries such as Belgium, Denmark, France and the Netherlands, according to the operator.
However, some countries such as Denmark, France and the Netherlands, have no longer received any Russian gas because they refused to pay their bills in rubles.