Fitch Ratings has lowered Russia's long-term foreign currency rating to C from B, in its second rating downgrade in less than a week. 

The C rating reflects the view that sovereign default is imminent, the global rating agency said in a statement late Tuesday.

Fitch warned that the failure to fulfill commercial debt payments could lead to further negative rating action.

"More generally, the further ratcheting up of sanctions, and proposals that could limit trade in energy, increase the probability of a policy response by Russia that includes at least selective non-payment of its sovereign debt obligations," it said.

Fitch announced on March 2 that it downgraded Russia's rating to B from BBB, saying its ratings have been placed on rating watch negative./aa