Russian natural gas supplies to Germany via the Baltic Sea pipeline Nord Stream 1 have been halted for 10 days for annual maintenance work.
But German Economy Minister Robert Habeck warned that EU countries had to be prepared in case gas shipments did not resume.
He has accused the Kremlin of using gas "as a weapon" in response to EU sanctions over the war in Ukraine.
Mr Habeck admitted Germany had become too dependent on Russian gas.
He described that as "a grave political mistake as we can see today, which we are trying to remedy as quickly as we possibly can".
He said two floating terminals for deliveries of liquefied natural gas (LNG) would be ready by the end of the year.
In mid-June Russia's state gas firm Gazprom cut gas flows through Nord Stream 1 to just 40% of the pipeline's capacity. It blamed a delay in the return of equipment being serviced by Germany's Siemens Energy.
The Canadian government says it will now return a repaired Siemens turbine to Germany for the pipeline. That move angered the Ukrainian government, which accused Canada of adjusting the sanctions imposed on Moscow "to the whims of Russia".
Canada says it is granting Siemens Canada a "time-limited and revocable permit" to send repaired turbines back to Germany, despite the sanctions.
Germany's government is worried that gas supplies could be reduced or cut permanently.
Pipeline maintenance is normal every summer, when gas demand is lower than in winter, but the worry is that Russia may not turn the taps back on.
The pipeline shutdown is also affecting Italy, where energy group Eni said it would receive about a third less gas from Russia's Gazprom on Monday compared with average volumes supplied over the past few days.
The head of the International Energy Agency, Fatih Birol, has warned that Russia may cut off gas supplies to Europe entirely and that Europe needs to prepare now.
Russia has already cut gas supplies to Poland, Bulgaria, the Netherlands, Denmark and Finland over their refusal to comply with a new payment scheme.
Austria and the Czech Republic get some gas from Nord Stream 1, but Russian gas also flows to them via a Ukraine pipeline.
Visiting Prague, Mr Habeck signed an agreement with the Czech Republic pledging mutual help in the event of a sharp drop in Russian gas supplies.
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