Russian state-owned energy giant Gazprom has told European buyers it may not be able to continue sending gas because of "extraordinary circumstances," according to an exclusive report by Reuters, raising fears that the ongoing annual maintenance of a key pipeline might extend beyond Thursday, the day it is expected to complete.

In a letter dated July 14 and seen by Reuters, Gazprom said it was retroactively declaring force majeure on supplies from June 14. The company cut the capacity of Nord Stream 1, which runs from Russia to Germany, to 40 percent on that date, blaming Western sanctions against Moscow for the reduction.

Nord Stream 1, which is undergoing maintenance, has not been delivering gas since Monday last week. If the maintenance doesn't complete as expected, many European Union countries that rely on Russian gas could be in a deeper energy crisis.