Russia's state-run energy giant Gazprom says it resumed deliveries of Russian gas to Germany on Thursday after suspension for maintenance work.

German officials remain wary as the supply remains at the same level as before the suspension, or 60 percent lower than normal.

Gazprom had said it would shut down the Nord Stream 1 pipeline connecting the two countries from July 11 to 21, citing annual maintenance work as the reason.

German officials were concerned that Russia might not reopen the pipeline as scheduled in retaliation for Germany's economic sanctions against Moscow.

The chief of the German energy regulator was not optimistic despite the resumption.

Klaus Mueller, the president of the Federal Network Agency, wrote on Twitter that gas flows on Thursday will remain about 60 percent lower than the normal level.

Gazprom reduced the amount of natural gas sent via the pipeline to Germany in June.

Concerns are growing in Germany as to whether it will be possible to secure sufficient supply when demand for gas increases in the winter.