Existing U.S. sanctions already ban U.S. investors from buying into new OFZ bonds. Some U.S. lawmakers recently suggested extending the restrictions on the secondary market, amid rising U.S. political tensions with Russia over everything from the Nord Stream 2 gas pipeline to relations with Ukraine.

"The Russian market, with such players as Sberbank and VTB, major OFZ holders, is incomparable with the OFZ portfolio held by U.S. investors," VTB CEO Andrey Kostin, a former diplomat, told Reuters.

Foreign investors hold 21% of total OFZ bonds outstanding, or 3.3 trillion roubles ($44 billion), of which U.S. investors account for around a third - less than VTB alone which has 1.75 trillion roubles of OFZs in its portfolio.

($1 = 74.9840 roubles)

(Reporting by Katya Golubkova; Editing by Susan Fenton)

By Katya Golubkova