MOSCOW, Feb 13 - Senior Russian officials have challenged the viability of U.S. proposals to build a nuclear power plant in Armenia, in a sign of Moscow's concern about the risk of losing a lucrative energy deal in a country it sees as part of its sphere of influence.

U.S. Vice President JD Vance, on a visit to Armenia this week, signed an agreement that could pave the way for the plant's construction and open what Vance called "a new chapter in the deepening partnership between Armenia and the United States".