MOSCOW — Sistema and its owner had for years assiduously steered clear of politics in Russia, hoping to avoid the kind of legal issues that often dog high-profile businesses here. But as it has grown into the country’s largest privately owned conglomerate, it has found itself mired in a multi-billion-dollar legal battle.

The dispute between Sistema and Rosneft, Russia’s state oil company, dates back to a 2014 nationalization, pitting two of the country’s most important companies against each other. The latest twist in their back-and-forth came on Monday, as Sistema warned banks about problems it is having repaying $66 million worth of loans.

The conflict between the two companies is another worrying sign for the investment climate in Russia. As the overall economy stagnates amid sanctions and low oil prices, the environment has raised the specter of the country’s elite fighting over a smaller pie, with businesses using courts and law enforcement that are widely seen as vulnerable to corruption to gain an advantage.

Sistema and its principal owner, Vladimir P. Yevtushenkov, have stayed away from politics — in one case, the company smoothly navigated the ouster of a former mayor, even though many of its assets are in the city of Moscow, staying aloof from the political struggle. It now employs about 150,000 people, and it says its revenues are equivalent to about 1 percent of Russia’s gross domestic product.