The head of EON, Germany's leading power supplier, warned Berlin Sunday against "recklessly" damaging ties with Russia over the crisis in Ukraine. Johannes Teyssen told news weekly Der Spiegel that the tensions risked hobbling a fruitful "partnership". "I don't want to interfere in foreign policy questions but I think that we have exercised very responsible policy toward the east in the last few decades," he was quoted as saying. "The build-up of trust and the interlocking of our economies have led to the fact that more than 6,000 German companies are active in Russia." He said that at the same time, Russian companies had invested heavily in Germany, particularly in the gas sector. "Through this partnership, our continent has grown more peaceful. That should not be recklessly jeopardised." EON has invested nearly six billion euros ($8.3 billion) in power plants in Russia, making it the biggest foreign energy supplier in the country. It is also a major importer of Russian gas. A poll published Sunday in the Bild am Sonntag newspaper indicated German voters were also sceptical about harsh sanctions against Russia over its seizure of the Ukraine's Crimean peninsula. Only one in five said they supported the sanctions the European Union has threatened to impose after Sunday's referendum in Crimea on breaking away from Ukraine. Forty-two percent told independent polling institute Emnid that they supported a "diplomatic" reaction, and 27 percent said the West should accept the results of the referendum. A clear majority -- 57 percent -- said they feared a harmful impact for German jobs and economic growth from the tensions with Russia, which supplies more than one-third of German gas and oil. One in three said Germany should postpone its gradual phase-out of nuclear power, due to be completed in 2022, to reduce dependence on Russian energy, while 59 percent opposed such a move.