The looming default is the result of sanctions that have immobilized about half of Russia’s $640 billion of foreign currency reserves, straining the country’s ability to make bond repayments in the currency the debt was issued in — dollars. Girding for a default, Russia has already pre-emptively dismissed it as an “artificial” result of sanctions imposed by the United States and its allies, and it has threatened to contest such an outcome in court.

The coming fight, which would probably pit Russia against big investors from around the world, raises murky questions over who gets to decide if a nation has actually defaulted in the rare case where sanctions have curbed a country’s ability to pay its debts.

Russia does not appear likely to take the declaration of a default lightly. If that should occur, it would raise Russia’s cost of borrowing for years to come and effectively lock it out of international capital markets, weighing on an economy that is already expected to contract sharply this year. It would also be a stain on the economic stewardship of President Vladimir V. Putin that would underscore the costs Russia is incurring from its invasion of Ukraine.