FRANKFURT — One of Germany’s biggest companies said Monday that it had become an unwitting pawn in a scheme to evade sanctions against Russia and break a de facto blockade of electricity to the annexed territory Crimea.

The company, Siemens, a giant engineering and electronics conglomerate based in Munich, said a Russian customer had illegally shipped two power plant turbines to Crimea instead of their intended destination in southern Russia. The diversion of the turbines flouted what Siemens said was an agreement not to violate sanctions imposed by the international community after Russia annexed the territory from Ukraine in 2014.

The incident threatens to strain relations between the countries, just days after Chancellor Angela Merkel of Germany hosted a contentious meeting of world leaders in Hamburg, attended by President Vladimir V. Putin of Russia. The Russian customer, Technopromexport, has close ties to the Kremlin.

The incident also demonstrates how energy has become a weapon in Russia’s continuing struggle with Ukraine, Crimea’s main source of electricity until the conflict interrupted supplies. Moscow had apparently become so desperate to solve an acute power shortage that it was willing to risk inflaming tensions with Germany.